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/models.py

@@ -8,6 +8,7 @@ from __future__ import annotations
 
 import datetime as dt
 import uuid
+from typing import Any
 
 from sqlalchemy import (
     BigInteger,
@@ -288,6 +289,23 @@ class SavedSearch(Base):
     pinned: Mapped[bool] = mapped_column(nullable=False, server_default=text("true"))
 
 
+class PluginKv(Base):
+    """Namespaced key/value store for plugins (SPEC §11 ``plugin_kv``).
+
+    Core-owned infrastructure (not a ``plg_``-prefixed plugin table): every plugin
+    gets a slice keyed by (plugin, user_id, key). ``value`` is arbitrary JSON.
+    Invariant I-7 (user-data rows carry ``user_id``) holds here too."""
+
+    __tablename__ = "plugin_kv"
+
+    plugin: Mapped[str] = mapped_column(Text, primary_key=True)
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), primary_key=True
+    )
+    key: Mapped[str] = mapped_column(Text, primary_key=True)
+    value: Mapped[Any] = mapped_column(JSONB, nullable=False)
+
+
 class KnownRecipient(Base):
     __tablename__ = "known_recipients"
     __table_args__ = (