[Agent] Add a license plate (LPR) search UI #653

Problem and evidence

ONVIF license-plate recognition reads are stored and fully exposed by the API (added in 0e091c6c), but the web UI has no screen for them. Nothing in web/js calls /api/lpr.

Backend (src/web/api_handlers_lpr.c, routes in src/web/libuv_api_handlers.c:289-291):

• POST /api/lpr/search: JSON body {camera_uuid (required), start_at, end_at (ms epoch, range ≤ 366 days), limit (default 50), match?: "exact"|"partial", plate?}. Returns reads with id, camera_uuid, stream_name, observed_at, received_at, source, plate, confidence, attributes{country, region, plate_type, direction, lane, vehicle_type, vehicle_color, ...}, recording_id?.
• POST /api/lpr/export: same body (limit default 1000). Responds with a JSON download (lightnvr-lpr-export.json).
• DELETE /api/lpr/reads/{uuid}.
• Authorization is per camera: AUTHZ_LPR_READ, AUTHZ_LPR_SEARCH, AUTHZ_LPR_EXPORT, AUTHZ_LPR_DELETE (include/core/authorization.h). Every query is audit-logged, and plate strings are secure_zero'd server side, so treat plates as sensitive data.

Expected behavior and scope

• Add a "Plates" (LPR) view. The Investigation workspace (web/js/components/preact/investigation/) is a natural home, either as a tab or as a sibling page linked from the Recordings/Timeline sub-nav. Choose whichever fits the existing nav, and explain the choice in the PR.
• Filters: camera picker (cameras the user can see, which supply camera_uuid), time range, plate text with an exact/partial toggle, and a limit.
• Results table: plate, time (local), camera, confidence, direction, vehicle type and color, and a "View recording" link using recording_id / observed_at that deep-links to the timeline at that time.
• Export button, shown only when export is permitted (a 403 should hide or disable it, not show an error toast). Delete per row, with confirmation, shown only when permitted.
• Empty state that explains LPR reads come from ONVIF cameras with plate analytics.
• All strings go through t() with new lpr.* keys in web/public/locales/en.json.
• Plates must not go into localStorage or URLs. Keep filter state in memory.

Acceptance criteria

• A user with search permission can find reads by partial plate within a time range and open the matching recording.
• A user without export or delete permission doesn't see those actions, or sees them disabled.
• 400 errors (bad range, etc.) show readable validation messages.
• Keyboard accessible and labelled (the repo runs jsx-a11y lint in CI).

Validation

• cd web && npx vite build, npm run lint, npm test.
• Jest tests for the request body built from the filters and for permission-driven rendering, with fetch mocked.
• Optional: an e2e spec that seeds reads through the DB test helpers, if any exist.

Constraints and rollback

• UI only. No backend changes needed. If you find an API gap (for example, listing cameras with LPR data), open a separate issue rather than changing the API here.
• Rollback: revert the frontend PR.