Replace global lazy singletons with explicit AppContainer + lifespan ...

- Create navi/core/container.py with AppContainer dataclass and create_container()
- Rewrite navi/api/deps.py: remove module-level singletons, add _container global
  fallback + set_container(), use _resolve_container() for all getters
- Replace @app.on_event with @asynccontextmanager lifespan in main.py
- Update routes to use Depends(get_scheduler) instead of calling get_scheduler()
- Fix FastAPI body parsing bug: remove dataclass parameters from Depends getters
  (FastAPI was treating AppContainer sub-dependencies as Body params, forcing
  embed=True on all endpoint body params and causing 422 errors)
- Update websocket.py to use _resolve_container() instead of get_registries()
- Update integration test fixtures to build AppContainer and call set_container()
- Remove obsolete tests/unit/test_startup.py (tests removed _on_startup)
- Fix test_scheduler_loop.py fixture (get_registries no longer exists)

All 387 tests pass (excluding websocket hang tests).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add pagination, search, and sorting to admin sessions ...

Backend:
- Add count_all and search_list abstract methods to SessionStore
- Implement count_all and search_list in PgSessionStore (SQL with ILIKE)
- Implement count_all and search_list in InMemorySessionStore
- Update /admin/sessions to accept limit, offset, search, sort_by, sort_order
- Return {total, limit, offset, items} from /admin/sessions

Frontend:
- Add search input for sessions in admin panel
- Add clickable sortable column headers with asc/desc toggle
- Add pagination controls (prev/next, page size selector, item count)
- Debounce search input (300ms)

Tests:
- Add integration tests for pagination, offset, search, and sorting
- All 217 tests pass

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Paginate session list loading

Add 62 tests across all planned phases + fix integration flakiness ...

- Phase 3: 19 API route integration tests (health, agents, sessions, messages)
- Phase 3: 7 WebSocket integration tests (connect, send, replay, invalid, stop)
- Phase 4: 9 agent tests (_check_context_size, _iter_stream_guarded)
- Phase 4: 5 planning tests (_parse_plan_steps)
- Phase 5: 22 tool tests (filesystem 13, code_exec 5, terminal 4)
- Fix flaky integration tests by patching module-level singletons
  (_session_store, _registries, _workers) instead of getter functions,
  because FastAPI Depends() captures the original function at import time.
- Update docs/testing.md coverage table (150 total tests)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>