refactor: move webapp from /user/page to /app

Simplified the webapp routing structure by consolidating the admin UI
to a single clean endpoint.

Changes:
- Moved webapp from /user/page to /app (root of mount)
- Removed /user JSON endpoint (no longer needed)
- Updated mount point from /user to /app in app.py
- Updated all route path checks (3 locations)
- Updated OAuth redirects to point to /app
- Updated all HTMX endpoint references
- Updated documentation (ADR-007, CHANGELOG)
- Added redirect from /app to /app/ for trailing slash handling

New Route Structure:
- /app - Main webapp (HTML UI with tabs)
- /app/revoke - Revoke background access
- /app/webhooks - Webhook management UI
- /app/webhooks/enable/{preset_id} - Enable webhook preset
- /app/webhooks/disable/{preset_id} - Disable webhook preset

Breaking Change: Existing bookmarks to /user or /user/page will no longer work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/ADR-007-background-vector-sync-job-management.md

@@ -377,7 +377,7 @@ async def get_vector_sync_status(ctx: Context) -> dict:
     }
 ```
 
-The web UI (`/user/page` route) mirrors these controls with a simple toggle switch for enabling/disabling sync and a status display showing indexed counts and sync state. There is no job history, no detailed progress bars, no per-document status—just the essential information users need.
+The web UI (`/app` route) mirrors these controls with a simple toggle switch for enabling/disabling sync and a status display showing indexed counts and sync state. There is no job history, no detailed progress bars, no per-document status—just the essential information users need.
 
 ### Authentication and Offline Access