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/plugins/alembic/env.py

@@ -0,0 +1,39 @@
+"""Shared Alembic environment for plugin migration branches (SPEC §11.1).
+
+Every plugin reuses this env; the runner (:mod:`app.plugins.migrate`) points
+``version_locations`` at the plugin's own revision directory and gives it a
+dedicated ``version_table`` (``alembic_version_plg_{name}``) so each plugin's
+migration history is independent of core and of every other plugin.
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+from alembic import context
+from sqlalchemy.ext.asyncio import async_engine_from_config
+from sqlalchemy.pool import NullPool
+
+from app.config import get_settings
+
+config = context.config
+config.set_main_option("sqlalchemy.url", get_settings().database_url)
+_version_table = config.get_main_option("version_table") or "alembic_version"
+
+
+def _do_run_migrations(connection: object) -> None:
+    context.configure(connection=connection, version_table=_version_table)  # type: ignore[arg-type]
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def _run_online() -> None:
+    engine = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}), prefix="sqlalchemy.", poolclass=NullPool
+    )
+    async with engine.connect() as connection:
+        await connection.run_sync(_do_run_migrations)
+    await engine.dispose()
+
+
+asyncio.run(_run_online())

backend/app/plugins/alembic/script.py.mako

@@ -0,0 +1,26 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+"""
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+revision: str = ${repr(up_revision)}
+down_revision: str | None = ${repr(down_revision)}
+branch_labels: str | Sequence[str] | None = ${repr(branch_labels)}
+depends_on: str | Sequence[str] | None = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}