feat(03-02): extend StorageBackend ABC and MinIOBackend with presigned PUT and stat_object

- Add generate_presigned_put_url and stat_object abstract methods to StorageBackend ABC
- Extend MinIOBackend with dual client (self._client internal + self._public_client public)
- MinIOBackend.__init__ accepts optional public_endpoint param (RESEARCH.md Finding 3)
- generate_presigned_put_url uses self._public_client for browser-resolvable URLs
- stat_object uses self._client.stat_object and returns .size (authoritative, T-03-05)
- get_storage_backend() passes public_endpoint=settings.minio_public_endpoint
- config.py adds minio_public_endpoint field (RESEARCH.md Finding 3)
- docker-compose.yml: MINIO_API_CORS_ALLOW_ORIGIN on minio service (T-03-09)
- docker-compose.yml: MINIO_PUBLIC_ENDPOINT on backend service
- docker-compose.yml: new celery-beat service (RESEARCH.md Finding 10)

backend/config.py

@@ -25,6 +25,9 @@ class Settings(BaseSettings):
     minio_access_key: str = "docuvault_app"
     minio_secret_key: str = "changeme_minio_app"
     minio_bucket: str = "docuvault"
+    # RESEARCH.md Finding 3 — browser-resolvable hostname for presigned URLs.
+    # Empty string means fall back to minio_endpoint inside MinIOBackend.
+    minio_public_endpoint: str = ""
 
     # Redis / Celery
     redis_url: str = "redis://:changeme_redis@redis:6379/0"

backend/storage/__init__.py

@@ -17,11 +17,17 @@ def get_storage_backend() -> StorageBackend:
 
     secure=False is correct for Docker internal HTTP traffic between containers
     (RESEARCH.md Pattern 3).
+
+    public_endpoint is the browser-resolvable hostname for presigned PUT URLs.
+    RESEARCH.md Finding 3 — dual-client pattern: internal endpoint for all
+    server-side operations; public endpoint for generate_presigned_put_url only.
     """
+    public_ep = settings.minio_public_endpoint or None
     return MinIOBackend(
         endpoint=settings.minio_endpoint,
         access_key=settings.minio_access_key,
         secret_key=settings.minio_secret_key,
         bucket=settings.minio_bucket,
         secure=False,
+        public_endpoint=public_ep,
     )

backend/storage/base.py

@@ -4,12 +4,14 @@ StorageBackend ABC for DocuVault.
 Mirrors backend/ai/base.py — declares the abstract interface that all storage
 backends (MinIO, OneDrive, Google Drive, Nextcloud, WebDAV) must implement.
 
-Five abstract methods define the contract:
+Seven abstract methods define the contract:
   put_object               — store bytes, return object key
   get_object               — fetch bytes by key
   delete_object            — remove object by key
   presigned_get_url        — generate a time-limited download URL
   health_check             — verify backend connectivity
+  generate_presigned_put_url — generate a presigned PUT URL for direct browser upload
+  stat_object              — return authoritative file size from object storage
 """
 from abc import ABC, abstractmethod
 
@@ -52,3 +54,28 @@ class StorageBackend(ABC):
     async def health_check(self) -> bool:
         """Return True if the backend is reachable and operational."""
         ...
+
+    @abstractmethod
+    async def generate_presigned_put_url(
+        self, object_key: str, expires_minutes: int = 15
+    ) -> str:
+        """Return a presigned PUT URL for direct browser-to-storage upload.
+
+        RESEARCH.md Finding 3 — public client requirement: the returned URL must
+        use a browser-resolvable hostname (not the internal Docker hostname).
+        The presigned URL is tied to the exact object_key and expires after
+        expires_minutes (default 15 — D-05).
+        """
+        ...
+
+    @abstractmethod
+    async def stat_object(self, object_key: str) -> int:
+        """Return the authoritative file size in bytes for the given object.
+
+        RESEARCH.md Finding 5 — returns the size (.size attribute) from the
+        object storage stat call. This is the only trusted source of file size;
+        never use client-supplied size values (D-07, T-03-05).
+
+        Raises S3Error(code='NoSuchKey') if the object does not exist.
+        """
+        ...

backend/storage/minio_backend.py

@@ -11,10 +11,13 @@ The human-readable filename is NEVER passed into this module — only the
 file extension (derived by the caller from Path(original_name).suffix.lower())
 reaches here.
 """
+from __future__ import annotations
+
 import asyncio
 import io
 import uuid
 from datetime import timedelta
+from typing import Optional
 
 from minio import Minio
 
@@ -35,6 +38,7 @@ class MinIOBackend(StorageBackend):
         secret_key: str,
         bucket: str,
         secure: bool = False,
+        public_endpoint: Optional[str] = None,
     ) -> None:
         self._bucket = bucket
         self._client = Minio(
@@ -43,6 +47,15 @@ class MinIOBackend(StorageBackend):
             secret_key=secret_key,
             secure=secure,  # False for Docker internal HTTP traffic between containers
         )
+        # Second client for presigned URL generation — uses browser-accessible hostname.
+        # Falls back to internal client endpoint if not configured.
+        # RESEARCH.md Finding 3 — dual-client pattern to avoid Docker hostname pitfall (T-03-10).
+        self._public_client = Minio(
+            endpoint=(public_endpoint or endpoint),
+            access_key=access_key,
+            secret_key=secret_key,
+            secure=secure,
+        )
 
     async def put_object(
         self,
@@ -104,3 +117,32 @@ class MinIOBackend(StorageBackend):
             return await asyncio.to_thread(self._client.bucket_exists, self._bucket)
         except Exception:
             return False
+
+    async def generate_presigned_put_url(
+        self, object_key: str, expires_minutes: int = 15
+    ) -> str:
+        """Return a presigned PUT URL using the public-endpoint client.
+
+        Uses self._public_client so the returned URL contains a browser-resolvable
+        hostname (not the Docker-internal 'minio:9000' address).
+        RESEARCH.md Finding 2: presigned_put_object(bucket, key, expires=timedelta).
+        RESEARCH.md Finding 3: dual-client pattern for Docker hostname pitfall (T-03-10).
+        """
+        return await asyncio.to_thread(
+            self._public_client.presigned_put_object,
+            self._bucket,
+            object_key,
+            timedelta(minutes=expires_minutes),
+        )
+
+    async def stat_object(self, object_key: str) -> int:
+        """Return the authoritative file size in bytes from MinIO stat.
+
+        Calls self._client.stat_object (internal endpoint) and returns .size.
+        RESEARCH.md Finding 5: stat_object returns .size as int (authoritative).
+        Raises minio.error.S3Error(code='NoSuchKey') if the object does not exist.
+        """
+        result = await asyncio.to_thread(
+            self._client.stat_object, self._bucket, object_key
+        )
+        return result.size

docker-compose.yml

@@ -21,6 +21,8 @@ services:
     environment:
       MINIO_ROOT_USER: ${MINIO_ROOT_USER}
       MINIO_ROOT_PASSWORD: ${MINIO_ROOT_PASSWORD}
+      # RESEARCH.md Finding 3, T-03-09: allow browser CORS preflight for direct PUT uploads.
+      MINIO_API_CORS_ALLOW_ORIGIN: ${CORS_ORIGINS:-http://localhost:5173}
     ports:
       - "9000:9000"
       - "9001:9001"
@@ -55,6 +57,7 @@ services:
       - MINIO_ACCESS_KEY=${MINIO_ACCESS_KEY}
       - MINIO_SECRET_KEY=${MINIO_SECRET_KEY}
       - MINIO_BUCKET=${MINIO_BUCKET}
+      - MINIO_PUBLIC_ENDPOINT=${MINIO_PUBLIC_ENDPOINT:-localhost:9000}
       - REDIS_URL=${REDIS_URL}
       - PYTHONDONTWRITEBYTECODE=1
     extra_hosts:
@@ -89,6 +92,27 @@ services:
       redis:
         condition: service_healthy
 
+  celery-beat:
+    build: ./backend
+    environment:
+      - DATABASE_URL=${DATABASE_URL}
+      - MINIO_ENDPOINT=${MINIO_ENDPOINT}
+      - MINIO_ACCESS_KEY=${MINIO_ACCESS_KEY}
+      - MINIO_SECRET_KEY=${MINIO_SECRET_KEY}
+      - MINIO_BUCKET=${MINIO_BUCKET}
+      - REDIS_URL=${REDIS_URL}
+      - PYTHONDONTWRITEBYTECODE=1
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
+    command: celery -A celery_app beat --loglevel=info
+    depends_on:
+      postgres:
+        condition: service_healthy
+      minio:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+
   frontend:
     build: ./frontend
     ports: