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/sdk.py

@@ -0,0 +1,116 @@
+"""The plugin SDK — the ONLY ``app.*`` surface a plugin may import (SPEC §11).
+
+Everything a plugin needs is reached through :func:`sdk`, which returns a
+:class:`PluginSdk` bound to the plugin's name: a plugin-scoped DB session, the
+event bus (publishing under ``plugin.{name}.*``), the blob store, outbox
+draft/send helpers, the settings accessor, an SSRF-guarded HTTP client
+(§10.6), and the namespaced KV store (§11).
+
+The frozen contract types (:class:`Plugin`, :class:`UiManifest`, :class:`JobSpec`,
+…) are re-exported here so plugin packages import only from ``app.plugins.sdk``.
+"""
+
+from __future__ import annotations
+
+import uuid
+from contextlib import AbstractAsyncContextManager
+from dataclasses import dataclass
+from typing import Any
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.accounts.ssrf import guarded_get
+from app.config import Settings, get_settings
+from app.events import get_event_bus
+from app.indexer import blobstore
+from app.outbox import service as outbox_service
+from app.plugins.db import plugin_session
+from app.plugins.kv import PluginKvStore
+from app.plugins.types import (
+    Hook,
+    JobFunc,
+    JobSpec,
+    Page,
+    PageNav,
+    Plugin,
+    Slot,
+    UiManifest,
+)
+from app.queue import enqueue_job
+
+__all__ = [
+    "Hook",
+    "JobFunc",
+    "JobSpec",
+    "Page",
+    "PageNav",
+    "Plugin",
+    "PluginSdk",
+    "Slot",
+    "UiManifest",
+    "blobstore",
+    "sdk",
+]
+
+
+@dataclass(frozen=True)
+class PluginSdk:
+    """A lightweight facade over shared services, scoped to one plugin.
+
+    Cheap to construct (wraps process-wide singletons + the plugin name), so
+    handlers/routes obtain it on demand via ``sdk("<name>")``."""
+
+    name: str
+
+    # --- config -------------------------------------------------------------
+    @property
+    def settings(self) -> Settings:
+        return get_settings()
+
+    # --- database -----------------------------------------------------------
+    def session(self, user_id: uuid.UUID) -> AbstractAsyncContextManager[AsyncSession]:
+        """An RLS-enforced transactional session scoped to ``user_id`` (SPEC §11.1).
+
+        Row-level security limits every query to this user's rows on ``plg_*``
+        tables — even a query that forgets ``WHERE user_id`` cannot leak or write
+        across users (see :mod:`app.plugins.db`)."""
+        return plugin_session(user_id)
+
+    def kv(self, user_id: uuid.UUID) -> PluginKvStore:
+        """The namespaced KV store for this plugin and ``user_id`` (SPEC §11)."""
+        return PluginKvStore(self.name, user_id)
+
+    # --- events -------------------------------------------------------------
+    async def publish(self, event: str, payload: dict[str, object]) -> None:
+        """Publish ``plugin.{name}.{event}`` on the bus (payload needs ``user_id``)."""
+        await get_event_bus().publish(f"plugin.{self.name}.{event}", payload)
+
+    # --- background jobs ----------------------------------------------------
+    async def enqueue(self, job_func: JobFunc, **kwargs: Any) -> None:
+        """Enqueue one of this plugin's jobs on the shared worker (SPEC §11.2).
+
+        ``job_func`` must be a job declared in this plugin's ``jobs=[...]``. Pass
+        ``user_id=`` for singleton jobs so the per-user lock applies."""
+        await enqueue_job(f"plg.{self.name}.{job_func.__name__}", **kwargs)
+
+    # --- blob store ---------------------------------------------------------
+    @property
+    def blob(self) -> object:
+        """The blob store (``store``/``load``/``exists``/``remove``), keyed by user."""
+        return blobstore
+
+    # --- outbox -------------------------------------------------------------
+    @property
+    def outbox(self) -> object:
+        """Draft/send helpers (``upsert_draft``, ``create_outbox``, …)."""
+        return outbox_service
+
+    # --- outbound HTTP ------------------------------------------------------
+    async def http_get(self, url: str) -> str | None:
+        """GET ``url`` through the §10.6 SSRF guard (blocks private IPs/oversize)."""
+        return await guarded_get(url)
+
+
+def sdk(name: str) -> PluginSdk:
+    """Return the SDK facade bound to plugin ``name`` (SPEC §11)."""
+    return PluginSdk(name)