feat: add V3 action endpoint for event exhibit tracking export

- New router: app/routers/api_v3_actions_event_exhibit.py
  - GET /v3/action/event_exhibit/{exhibit_id}/tracking_export
  - Full V3 auth (x-aether-api-key + account context)
  - Multi-tenant ownership check via check_account_access
  - Permission gate: leads_api_access flag OR manager-level access
  - Returns CSV or XLSX file attachment (return_file=false for JSON)
  - Flattens responses_json custom Q&A columns; strips HTML from exhibitor_notes
  - Exports all records regardless of hidden/enabled state

- Registered in registry.py under prefix /v3/action/event_exhibit

- New E2E test: tests/e2e/test_e2e_v3_action_event_exhibit_tracking_export.py
  - 7/7 tests passing against dev-api.oneskyit.com

- Docs: GUIDE__AE_API_V3_for_Frontend.md — new Section 7 covering endpoint
  usage, columns, leads_api_access dual-purpose (3rd-party API + UI export gate)

- Docs: tests/README.md — added test to table and when-to-run matrix

tests/README.md

@@ -28,6 +28,7 @@ These consolidated scripts are the primary verification tool for the V3 API.
 | `test_e2e_v3_demo_parity.py` | **Demo Parity + Nested Create Regression**: Vision ID check for Badge, Exhibit, Tracking; nested create lifecycle (POST+DELETE) for `journal/journal_entry` and `event/event_session`; alias resolution. **Run after any model or nested-router change.** |
 | `test_e2e_v3_action_event_file.py` | **Event Actions**: Specialized atomic upload and linking for event files. |
 | `test_e2e_v3_action_zoom.py` | **Zoom Integration**: Verifies OAuth and ticket sync logic for Zoom Events. |
+| `test_e2e_v3_action_event_exhibit_tracking_export.py` | **Exhibit Leads Export**: Auth/permission guards, CSV column structure, XLSX bytes, and `return_file` mode for the V3 tracking export action. |
 | `test_e2e_v3_accounts.py` | CRUD verification for the core Account object. |
 | `test_e2e_v3_schema.py` | Network verification of the V3 metadata discovery endpoint. |
 | `test_e2e_agent_bridge.py` | Verifies container diagnostics and log streaming routes. |
@@ -52,6 +53,7 @@ Tests exist to be used — run the relevant suite whenever you touch backend cod
 | Search / filter changes | `test_e2e_v3_search_engine.py` |
 | Auth / account context changes | `test_e2e_v3_security_audit.py`, `test_e2e_v3_auth_security.py` |
 | File upload / download changes | `test_e2e_v3_actions_file_lifecycle.py` |
+| Event exhibit tracking export changes | `test_e2e_v3_action_event_exhibit_tracking_export.py` |
 | Any backend change before frontend hand-off | All of the above |
 
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