M3 plugin platform: SDK + loader, RLS-isolated tables, migrations, jobs

Backend foundation for the plugin platform (SPEC §11), Phases 1-3.

Phase 1 — SDK + loader:
- app/plugins/types.py: frozen Plugin/UiManifest/Slot/Page/JobSpec contract.
- app/plugins/sdk.py: sdk(name) facade — the only app.* import allowed for plugins
  (scoped session, plugin.{name}.* publish, blob, outbox, settings, SSRF http_get, KV, enqueue).
- app/plugins/loader.py: entry-point discovery (webmail.plugins), hook forwarding
  (API + worker), router mounting at /api/plugins/{name}, ui-manifest aggregation.
- plugin_kv core table (migration 0013) + GET /api/plugins/ui-manifest.

Phase 2 — non-removable core plugins + DB-enforced privacy:
- builtin flag (calendar/contacts will be builtin=True; uninstall refuses them).
- RLS per-user isolation on plg_* tables: least-privilege webmail_plugin role
  (migration 0014), plugin_session(user_id) sets LOCAL ROLE + app.current_user_id,
  rls.enable_rls() (ENABLE+FORCE RLS + USING/WITH CHECK policy). Mail untouched.
- Per-plugin Alembic runner (shared env, dedicated version table) + install/uninstall
  --drop-data CLI; skeleton fixture proves install -> run -> uninstall --drop-data
  leaves zero residue (§12 M3 acceptance).

Phase 3 — plugin jobs (§11.2):
- JobSpec registered as arq function plg.{plugin}.{func} + cron (5-field parser);
  singleton=True guarded by a per-(plugin,user) Redis lock; wired onto the shared
  interactive worker.

Gates: ruff, mypy --strict (94 files + tests), full suite 326 passed. RLS isolation,
migration round-trip, and singleton lock covered by tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

backend/app/api/__init__.py

@@ -16,6 +16,7 @@ def register_routers(app: FastAPI) -> None:
     from app.api.compose import router as compose_router
     from app.api.mailboxes import router as mailboxes_router
     from app.api.messages import router as messages_router
+    from app.api.plugins import router as plugins_router
     from app.api.recipients import router as recipients_router
     from app.api.saved_searches import router as saved_searches_router
     from app.api.search import router as search_router
@@ -36,6 +37,13 @@ def register_routers(app: FastAPI) -> None:
         settings_router,
         mailboxes_router,
         saved_searches_router,
+        plugins_router,
     ):
         app.include_router(router, prefix="/api")
     app.include_router(ws_router, prefix="/api")
+
+    # Mount discovered plugin routers at /api/plugins/{name} (SPEC §11). Hooks are
+    # subscribed in the lifespan startup so they register once per process.
+    from app.plugins import loader
+
+    loader.mount_routers(app)

backend/app/api/plugins.py

@@ -0,0 +1,23 @@
+"""Plugin platform HTTP surface (SPEC §11).
+
+Currently just the aggregated UI manifest the frontend loader fetches on boot.
+Individual plugin routers are mounted separately at ``/api/plugins/{name}`` by
+:func:`app.plugins.loader.mount_routers`.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import APIRouter
+
+from app.identity.deps import CurrentUser
+from app.plugins import loader
+
+router = APIRouter(prefix="/plugins", tags=["plugins"])
+
+
+@router.get("/ui-manifest")
+async def ui_manifest(user: CurrentUser) -> dict[str, Any]:
+    """Slots, pages, and capabilities declared by all installed plugins (SPEC §11.3)."""
+    return loader.build_ui_manifest()