feat(08-02): create backend/api/schemas.py with CloudConnectionOut

- New cross-package Pydantic schemas module (D-10, CODE-08)
- CloudConnectionOut moved here from api/admin.py to eliminate coupling
- SEC-08: credentials_enc deliberately excluded; 7-field whitelist preserved
- field_validator coerce_id_to_str preserves UUID→str coercion behavior
- Original definition in api/admin.py left intact until plan 08-04

backend/api/schemas.py

@@ -0,0 +1,42 @@
+"""Cross-package Pydantic response models.
+
+Models here are used by 2+ API packages and cannot live in a single package
+without creating circular imports (D-10, RESEARCH.md Pitfall 3).
+"""
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel, field_validator
+
+
+class CloudConnectionOut(BaseModel):
+    """SEC-08: credentials_enc deliberately excluded from this response model.
+
+    Any admin or user endpoint returning CloudConnection ORM objects MUST use
+    this model to prevent accidental exposure of encrypted credentials.
+    Safe-by-default: whitelist of allowed fields (not blacklist).
+
+    Moved from api/admin.py to eliminate cross-package coupling between
+    api/cloud.py and api/admin.py (D-10, RESEARCH.md Pitfall 3).
+    Used by api/cloud.py and api/admin/ (after plan 08-04 admin split).
+
+    Note: id is declared as str and coerced via validator so UUID ORM values
+    serialize correctly without json_encoders.
+    """
+
+    id: str
+    provider: str
+    display_name: str
+    status: str
+    connected_at: datetime
+    server_url: Optional[str] = None
+    connection_username: Optional[str] = None
+    model_config = {"from_attributes": True}
+
+    @field_validator("id", mode="before")
+    @classmethod
+    def coerce_id_to_str(cls, v) -> str:
+        """Coerce UUID objects to str so the model validates from ORM instances."""
+        return str(v)