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>

CHANGELOG.md

@@ -86,7 +86,7 @@
 - implement ADR-009 - refactor semantic search to use generic semantic:read scope
 - implement MCP sampling for semantic search RAG (ADR-008)
 - add optional vector database and semantic search to helm chart
-- add vector sync processing status to /user/page endpoint
+- add vector sync processing status to /app endpoint
 - implement semantic search tool and fix vector sync issues (ADR-007 Phase 3)
 - implement vector sync scanner and processor (ADR-007 Phase 2)