feat(phase-4): add write_audit_log() async helper (flush-not-commit, never-raises)

- Creates backend/services/audit.py with write_audit_log() function
- Uses session.flush() not session.commit() per D-14 architectural requirement
- Catches and logs all exceptions (never re-raises) so audit failure is non-fatal
- Correct AuditLog ORM attribute metadata_ (not metadata) per models.py

backend/services/audit.py

@@ -0,0 +1,58 @@
+"""
+Audit log service helper for DocuVault — Phase 4.
+
+Provides write_audit_log(), a fire-and-forget helper called inline by API
+handlers after successful operations (D-14).
+
+Key architectural constraints:
+  - Uses session.flush() NOT session.commit() (D-14: the caller owns the
+    transaction; the audit entry is flushed within the same transaction but
+    the commit remains the caller's responsibility).
+  - NEVER raises — audit failure must not abort the primary operation.
+    Exceptions are logged at WARNING level and silently swallowed.
+"""
+from __future__ import annotations
+
+import logging
+import uuid
+from typing import Optional
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from db.models import AuditLog
+
+logger = logging.getLogger(__name__)
+
+
+async def write_audit_log(
+    session: AsyncSession,
+    event_type: str,
+    user_id: Optional[uuid.UUID],
+    actor_id: Optional[uuid.UUID],
+    resource_id: Optional[uuid.UUID],
+    ip_address: Optional[str],
+    metadata_: Optional[dict] = None,
+) -> None:
+    """Write an audit log entry within the caller's transaction.
+
+    Never raises — audit failure is non-fatal. Exceptions are caught and
+    logged as warnings so the primary operation is never aborted.
+
+    Uses session.flush() not session.commit() (D-14). The caller is responsible
+    for committing the transaction; this function merely ensures the audit row
+    is queued in the same unit of work.
+    """
+    try:
+        entry = AuditLog(
+            event_type=event_type,
+            user_id=user_id,
+            actor_id=actor_id,
+            resource_id=resource_id,
+            ip_address=ip_address,
+            metadata_=metadata_,
+        )
+        session.add(entry)
+        await session.flush()  # flush within handler's existing transaction, not commit
+    except Exception as exc:
+        logger.warning("audit log write failed: %s", exc)
+        # Do not re-raise — audit failure must never abort the primary operation