feat(auth): add admin user management dashboard

- Add UserProfile model (app/models.py) with per-user settings: display_name, daily_upload_limit, notes, is_blocked
- Add Alembic migration 013_add_user_profiles for the new table
- Add REST API at /api/admin/users/ with list, get, upsert (PUT), delete endpoints (admin-only)
- Add HTML template admin_users.html with Alpine.js: filterable user list, paginated table, edit/create modal, delete confirmation modal
- Add view handler at /admin/users (admin-only redirect guard)
- Register routers in app/api/__init__.py and app/views/__init__.py
- Add 'Users' link to admin nav dropdown in base.html (desktop + mobile)
- Add 27 tests covering auth, list, get, upsert, delete, and model constraints
- Register UserProfile in conftest.py model imports
- Document new endpoints in docs/API.md

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

app/models.py

@@ -170,3 +170,35 @@ class WebhookConfig(Base):
     description = Column(String, nullable=True)  # Optional human-readable description
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+
+class UserProfile(Base):
+    """Per-user profile for admin-managed settings in multi-user mode.
+
+    Each row corresponds to one authenticated user (identified by their
+    ``user_id``, which matches ``FileRecord.owner_id``).  The admin can
+    create or update profiles to override global defaults such as the
+    daily upload limit and to attach notes or block a user.
+    """
+
+    __tablename__ = "user_profiles"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    # Stable user identifier — matches FileRecord.owner_id (OAuth sub / email / username)
+    user_id = Column(String, unique=True, nullable=False, index=True)
+
+    # Optional human-readable display name set by the admin
+    display_name = Column(String, nullable=True)
+
+    # Per-user daily upload limit; NULL means "use global default"
+    daily_upload_limit = Column(Integer, nullable=True)
+
+    # Admin-only free-text notes about this user
+    notes = Column(Text, nullable=True)
+
+    # When True the user is prevented from uploading new documents
+    is_blocked = Column(Boolean, default=False, nullable=False)
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())