Add local auth: AuthProvider, mailer, sessions, /api/v1/auth

Pluggable AuthProvider interface with a local (email+password) implementation, and a Mailer interface (ConsoleMailer for dev, SMTPMailer for operators). The auth service owns registration, login, opaque session issuance, email verification, and password reset (which revokes prior sessions). Endpoints under /api/v1/auth; sessions are returned as a Bearer token and set as an HttpOnly cookie.

Replaces the temporary X-User-Id shim: get_current_user now resolves a real session (Bearer or cookie). The open user-bootstrap endpoint is gone (registration replaces it). App logging is configured so the ConsoleMailer's verification/reset links are visible to self-hosters. Verified end-to-end on the deploy target, including the email-verification flow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/integrations/auth/base.py

@@ -0,0 +1,24 @@
+"""AuthProvider interface.
+
+Operators enable any subset of providers (local, OIDC, social). A provider's
+job is narrow: verify a credential and return the matching User (or None).
+Session issuance, tokens, and registration live in the auth service and are
+provider-agnostic, so adding OIDC/social later (Phase 5) is additive.
+"""
+
+from abc import ABC, abstractmethod
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.user import User
+
+
+class AuthProvider(ABC):
+    name: str
+
+    @abstractmethod
+    async def authenticate(
+        self, session: AsyncSession, *, identifier: str, secret: str
+    ) -> User | None:
+        """Return the User if the credential is valid, else None."""
+        raise NotImplementedError