Phase 6: Admin portal

Adds is_site_admin-gated site administration: user management (list,
deactivate/reactivate, reset password, promote/demote), room management
(list all rooms including private ones, archive/unarchive, force-transfer
ownership), and an audit log of every admin action.

Backend: User.is_active (deactivation) and Room.is_archived (archive) are
new columns; AdminAuditLog is a new table matching ARCHITECTURE.md's
admin_audit_log design, written to in the same transaction as every
mutating admin action. require_site_admin (dependencies.py) gates all
/api/admin/* routes. get_current_user now rechecks is_active on every
request, so deactivating a user kills their already-open session
immediately, not just future logins. An admin can't deactivate or demote
their own account (the one self-lockout guard included). Archived rooms
drop out of the open-room browse list but stay readable for existing
members.

Frontend: new /admin route (AdminRoute guard, redirects non-admins to
/rooms) with a tabbed Users / Rooms / Audit log / Settings page, plus an
"Admin" link in the account menu for site admins.

Bot/integration management and system settings -- both listed in the
original issue -- are intentionally not here: bot management has nothing
to manage until Phase 7 builds the actual bot data model, and there's no
settings storage or concrete setting to configure yet. Settings has an
empty placeholder tab; bot management is deferred entirely to Phase 7.
Confirmed this scope cut with the repo owner before implementing.

New tests/test_admin.py (14 tests, full suite now 58/58) covers every
admin endpoint's permission gate, the self-action guards, deactivation's
immediate effect on an already-open session, and that every mutating
action produces exactly one audit log row.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/dependencies.py

@@ -18,7 +18,7 @@ async def get_current_user(
         raise HTTPException(status_code=401, detail="Not authenticated")
 
     user = await db.get(User, uuid.UUID(user_id))
-    if user is None:
+    if user is None or not user.is_active:
         request.session.clear()
         raise HTTPException(status_code=401, detail="Not authenticated")
 
@@ -46,3 +46,8 @@ async def require_room_role(
     if _ROLE_RANK[membership.role] < _ROLE_RANK[minimum]:
         raise HTTPException(status_code=403, detail="Insufficient room role")
     return membership
+
+
+def require_site_admin(user: User) -> None:
+    if not user.is_site_admin:
+        raise HTTPException(status_code=403, detail="Site admin required")