feat(ui): add user autocomplete widget for default_owner_id, user search API, and documentation

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/user.py

@@ -4,14 +4,23 @@ User-related API endpoints
 
 import logging
 from hashlib import md5
+from typing import Annotated
 
-from fastapi import APIRouter, HTTPException, Request
+from fastapi import APIRouter, Depends, HTTPException, Query, Request
+from sqlalchemy import func
+from sqlalchemy.orm import Session
+
+from app.auth import require_login
+from app.database import get_db
+from app.models import FileRecord
 
 # Set up logging
 logger = logging.getLogger(__name__)
 
 router = APIRouter()
 
+DbSession = Annotated[Session, Depends(get_db)]
+
 
 async def whoami_handler(request: Request):
     """
@@ -46,3 +55,30 @@ async def whoami(request: Request):
 @router.get("/auth/whoami")
 async def auth_whoami(request: Request):
     return await whoami_handler(request)
+
+
+@router.get("/users/search")
+@require_login
+def search_known_users(
+    db: DbSession,
+    q: str = Query("", description="Substring to match against known owner IDs"),
+    limit: int = Query(5, ge=1, le=20, description="Maximum number of results"),
+):
+    """
+    Search known user identifiers (owner_ids) from existing documents.
+
+    Returns distinct ``owner_id`` values from the files table that contain
+    the query string as a case-insensitive substring.  Results are limited
+    to at most ``limit`` entries (default 5).
+
+    This powers the autocomplete widget on the settings page for the
+    ``default_owner_id`` field.
+    """
+    base_query = db.query(FileRecord.owner_id).filter(FileRecord.owner_id.isnot(None)).distinct()
+
+    if q.strip():
+        base_query = base_query.filter(func.lower(FileRecord.owner_id).contains(q.strip().lower()))
+
+    results = base_query.order_by(FileRecord.owner_id).limit(limit).all()
+
+    return {"users": [row[0] for row in results]}