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/routers/auth.py

@@ -6,7 +6,11 @@ from app.dependencies import get_current_user
 from app.models import User
 from app.schemas.auth import LoginRequest
 from app.schemas.user import UserRead
-from app.services.auth_service import InvalidCredentialsError, authenticate_user
+from app.services.auth_service import (
+    AccountDeactivatedError,
+    InvalidCredentialsError,
+    authenticate_user,
+)
 
 # No POST /register here: this is an invite-only site. Accounts are created
 # by an operator via `python -m app.cli create-user` (see app/cli.py), not
@@ -25,6 +29,8 @@ async def login(
         )
     except InvalidCredentialsError:
         raise HTTPException(status_code=401, detail="Invalid username/email or password")
+    except AccountDeactivatedError:
+        raise HTTPException(status_code=401, detail="Account is deactivated")
 
     request.session["user_id"] = str(user.id)
     return user