M3 plugin platform: frontend host, reference plugins, UI kit + docs

Completes M3 (SPEC §11 + §12 acceptance) on top of the backend platform (4e077f1).

Frontend host (§11.3):
- src/plugins/: manifest loader, runtime ESM bundle loader (framework-agnostic
  mount contract), PluginSlot, PluginPage (app.page at /p/*), plugin context
  (scoped api/events/i18n), capabilities hook. Wired slots into ThreadView
  (message.actions/message.banner), MailLayout sidebar (sidebar.section + page nav)
  and Settings→Accounts (settings.page); ws.ts fans out core + plugin.* events.
- plugin-ui.css: theme-aware .wm-* classes so bundles don't re-derive inline styles.

SDK additions:
- CurrentUserId + require_csrf (plugin-router auth), messages.envelope/header,
  actions.set_flags/trash, sdk.enqueue.

Reference plugins (plugins/, installed via webmail.plugins entry points):
- hello: skeleton page-plugin (app.page + plg_hello_note w/ RLS + cron job) —
  install→run→uninstall --drop-data leaves zero residue.
- labels: message.actions + sidebar.section slots, KV only, zero core diffs.
- filters: message.new hook -> match (subject/from/to) -> Mark read / Pin / Delete
  (delete = move thread to Trash via the core path).

docs/plugins.md: full authoring guide + a complete "Unsubscribe button" worked
example (the §12 "writable from docs alone" bar).

Also: conftest resets the arq pool between tests (loop-bound global, like engine/redis).

Gates: backend ruff + mypy --strict (107) + full suite 337; frontend tsc + eslint
+ vitest (11) + vite build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

backend/app/plugins/sdk.py

@@ -15,15 +15,19 @@ from __future__ import annotations
 import uuid
 from contextlib import AbstractAsyncContextManager
 from dataclasses import dataclass
-from typing import Any
+from typing import Annotated, Any
 
+from fastapi import Depends
 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.identity.deps import current_user, require_csrf
 from app.indexer import blobstore
+from app.models import User
 from app.outbox import service as outbox_service
+from app.plugins.actions import PluginActions, PluginMessages
 from app.plugins.db import plugin_session
 from app.plugins.kv import PluginKvStore
 from app.plugins.types import (
@@ -39,6 +43,7 @@ from app.plugins.types import (
 from app.queue import enqueue_job
 
 __all__ = [
+    "CurrentUserId",
     "Hook",
     "JobFunc",
     "JobSpec",
@@ -49,10 +54,21 @@ __all__ = [
     "Slot",
     "UiManifest",
     "blobstore",
+    "require_csrf",
     "sdk",
 ]
 
 
+async def _current_user_id(user: Annotated[User, Depends(current_user)]) -> uuid.UUID:
+    return user.id
+
+
+# Auth for plugin routers (plugins must not import app.identity directly, SPEC §11):
+# `CurrentUserId` resolves the authenticated user's id; `require_csrf` guards mutating
+# routes. Use with the RLS-scoped `sdk(name).session(user_id)`.
+CurrentUserId = Annotated[uuid.UUID, Depends(_current_user_id)]
+
+
 @dataclass(frozen=True)
 class PluginSdk:
     """A lightweight facade over shared services, scoped to one plugin.
@@ -105,6 +121,17 @@ class PluginSdk:
         """Draft/send helpers (``upsert_draft``, ``create_outbox``, …)."""
         return outbox_service
 
+    # --- messages -----------------------------------------------------------
+    @property
+    def messages(self) -> PluginMessages:
+        """Read a user's message envelope (subject/sender/flags) for matching."""
+        return PluginMessages()
+
+    @property
+    def actions(self) -> PluginActions:
+        """User-scoped message actions (``set_flags``) — e.g. for filter rules."""
+        return PluginActions()
+
     # --- outbound HTTP ------------------------------------------------------
     async def http_get(self, url: str) -> str | None:
         """GET ``url`` through the §10.6 SSRF guard (blocks private IPs/oversize)."""