feat(slice-2): deps — mailer + session dependencies; build mailer at startup

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/platform/deps.py

@@ -1,18 +1,38 @@
 """FastAPI dependencies — per-request wiring (SD-0001 §6.2 platform/deps).
 
-Yields a pooled connection per request. The pool lives on app.state, built once in
-create_app(). Later slices add the current-session and mailer dependencies here.
+Yields a pooled connection per request, the process-wide mailer, and the verified session
+payload (from the signed cookie). The pool and mailer live on app.state, built once in
+create_app(). The storefronts dependency lands in SLICE-3.
 """
 from __future__ import annotations
 
 from collections.abc import Iterator
+from typing import Any
 
 import psycopg
 from fastapi import Request
 
+from app.platform import config, session
+from app.platform.mailer import Mailer
+
+SESSION_COOKIE = "ecomm_session"
+
 
 def get_conn(request: Request) -> Iterator[psycopg.Connection]:
     """A pooled connection for the duration of one request."""
     pool = request.app.state.pool
     with pool.connection() as conn:
         yield conn
+
+
+def get_mailer(request: Request) -> Mailer:
+    """The process-wide mailer adapter (built from config at startup, INV-8)."""
+    return request.app.state.mailer
+
+
+def get_session(request: Request) -> dict[str, Any] | None:
+    """The verified session payload from the cookie, or None if absent/invalid."""
+    token = request.cookies.get(SESSION_COOKIE)
+    if not token:
+        return None
+    return session.verify(token, config.session_secret())