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/tests/plugins/test_sdk.py

@@ -0,0 +1,51 @@
+"""Plugin SDK facade behaviour (SPEC §11)."""
+
+from __future__ import annotations
+
+import uuid
+
+import pytest
+
+from app.events import get_event_bus
+from app.plugins.sdk import sdk
+
+
+async def test_publish_namespaces_under_plugin_name() -> None:
+    received: list[dict[str, object]] = []
+
+    async def handler(payload: dict[str, object]) -> None:
+        received.append(payload)
+
+    event = f"plugin.demo.thing_{uuid.uuid4().hex[:6]}"
+    suffix = event.removeprefix("plugin.demo.")
+    get_event_bus().subscribe(event, handler)
+
+    await sdk("demo").publish(suffix, {"user_id": "u1", "x": 1})
+    assert received == [{"user_id": "u1", "x": 1}]
+
+
+async def test_publish_requires_user_id() -> None:
+    with pytest.raises(ValueError, match="user_id"):
+        await sdk("demo").publish("thing", {"x": 1})
+
+
+async def test_http_get_uses_ssrf_guard(monkeypatch: pytest.MonkeyPatch) -> None:
+    calls: list[str] = []
+
+    async def fake_guarded_get(url: str) -> str | None:
+        calls.append(url)
+        return f"body:{url}"
+
+    monkeypatch.setattr("app.plugins.sdk.guarded_get", fake_guarded_get)
+    out = await sdk("demo").http_get("http://example.test/x")
+    assert out == "body:http://example.test/x"
+    assert calls == ["http://example.test/x"]
+
+
+def test_sdk_exposes_shared_services() -> None:
+    s = sdk("demo")
+    assert s.name == "demo"
+    assert s.settings is not None
+    # blob store + outbox helpers are reachable through the SDK namespace.
+    assert hasattr(s.blob, "store") and hasattr(s.blob, "load")
+    assert hasattr(s.outbox, "upsert_draft")