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/loader.py

@@ -0,0 +1,148 @@
+"""Plugin discovery + core wiring (SPEC §11).
+
+Discovers plugins via the ``webmail.plugins`` entry-point group, then:
+- mounts each plugin router at ``/api/plugins/{name}`` (API process),
+- forwards §8.3 events to plugin hooks (both API and worker processes),
+- aggregates UI manifests for ``GET /api/plugins/ui-manifest``.
+
+Plugin background jobs (§11.2) and migrations (§11.1) are wired in later M3 phases.
+"""
+
+from __future__ import annotations
+
+import logging
+from importlib.metadata import entry_points
+from typing import TYPE_CHECKING, Any
+
+from app.events import EventBus
+from app.plugins.types import Page, Plugin, Slot, UiManifest
+
+if TYPE_CHECKING:
+    from fastapi import FastAPI
+
+logger = logging.getLogger(__name__)
+
+ENTRYPOINT_GROUP = "webmail.plugins"
+
+_plugins: list[Plugin] | None = None
+# Plugin names whose hooks are already subscribed on a given bus, so repeated
+# create_app()/worker starts in one process don't double-register (tests, reload).
+_hooks_registered: set[tuple[int, str]] = set()
+
+
+def _coerce_plugin(obj: Any) -> Plugin:
+    """An entry point may resolve to a ``Plugin`` or a module exposing ``plugin``."""
+    if isinstance(obj, Plugin):
+        return obj
+    candidate = getattr(obj, "plugin", None)
+    if isinstance(candidate, Plugin):
+        return candidate
+    raise TypeError(f"entry point {obj!r} does not expose a Plugin (SPEC §11)")
+
+
+def discover_plugins() -> list[Plugin]:
+    """Load every plugin registered under the ``webmail.plugins`` group."""
+    found: list[Plugin] = []
+    seen: set[str] = set()
+    for ep in entry_points(group=ENTRYPOINT_GROUP):
+        try:
+            plugin = _coerce_plugin(ep.load())
+        except Exception:  # noqa: BLE001 - one bad plugin must not break the app
+            logger.exception("failed to load plugin entry point %s", ep.name)
+            continue
+        if plugin.name in seen:
+            logger.error("duplicate plugin name %r; skipping", plugin.name)
+            continue
+        seen.add(plugin.name)
+        found.append(plugin)
+        logger.info("loaded plugin %s v%s", plugin.name, plugin.version)
+    return found
+
+
+def get_plugins() -> list[Plugin]:
+    """Return the process-wide discovered plugin list (cached)."""
+    global _plugins
+    if _plugins is None:
+        _plugins = discover_plugins()
+    return _plugins
+
+
+def set_plugins_for_test(plugins: list[Plugin] | None) -> None:
+    """Override (or reset with ``None``) the plugin registry — tests only."""
+    global _plugins
+    _plugins = plugins
+    _hooks_registered.clear()
+
+
+def register_hooks(bus: EventBus) -> None:
+    """Subscribe each plugin's hooks to the bus (idempotent per process/bus).
+
+    Called in both the API and the worker so hooks fire wherever the event is
+    published (e.g. ``message.new`` originates in the worker)."""
+    for plugin in get_plugins():
+        marker = (id(bus), plugin.name)
+        if marker in _hooks_registered:
+            continue
+        for event, handler in plugin.hooks.items():
+            bus.subscribe(event, handler)
+        _hooks_registered.add(marker)
+
+
+def mount_routers(app: FastAPI) -> None:
+    """Mount each plugin's router at ``/api/plugins/{name}`` (SPEC §11)."""
+    for plugin in get_plugins():
+        if plugin.router is not None:
+            app.include_router(plugin.router, prefix=f"/api/plugins/{plugin.name}")
+
+
+def _slot_dict(slot: Slot) -> dict[str, str]:
+    return {"name": slot.name, "bundle": slot.bundle}
+
+
+def _page_dict(page: Page) -> dict[str, Any]:
+    return {
+        "route": page.route,
+        "bundle": page.bundle,
+        "nav": {"icon": page.nav.icon, "label": page.nav.label, "order": page.nav.order},
+    }
+
+
+def _manifest_dict(plugin: Plugin, manifest: UiManifest) -> dict[str, Any]:
+    return {
+        "name": plugin.name,
+        "version": plugin.version,
+        "builtin": plugin.builtin,  # core plugins the UI must not offer to remove
+        "slots": [_slot_dict(s) for s in manifest.slots],
+        "pages": [_page_dict(p) for p in manifest.pages],
+        "capabilities": dict(manifest.capabilities),
+    }
+
+
+def build_ui_manifest() -> dict[str, Any]:
+    """Aggregate every plugin's UI manifest for the frontend loader (SPEC §11.3)."""
+    plugins = [
+        _manifest_dict(p, p.ui_manifest) for p in get_plugins() if p.ui_manifest is not None
+    ]
+    return {"plugins": plugins}
+
+
+def get_plugin(name: str) -> Plugin | None:
+    """Return the loaded plugin named ``name``, or None."""
+    return next((p for p in get_plugins() if p.name == name), None)
+
+
+class PluginRemovalError(Exception):
+    """Raised when trying to uninstall a builtin (core) plugin (SPEC §11 / §12)."""
+
+
+def assert_removable(name: str) -> Plugin:
+    """Return the plugin, or raise if it's unknown or builtin (non-removable).
+
+    Core plugins (calendar, contacts) ship ``builtin=True`` and may never be
+    uninstalled by a user; the uninstall path calls this before touching data."""
+    plugin = get_plugin(name)
+    if plugin is None:
+        raise PluginRemovalError(f"unknown plugin {name!r}")
+    if plugin.builtin:
+        raise PluginRemovalError(f"plugin {name!r} is builtin and cannot be removed")
+    return plugin