feat: phase 3 admin magic-link auth — tokens, sessions, rate limits, audit

End-to-end passwordless admin auth. /admin/login accepts an email, POSTs
mint a 256-bit magic-link token stored only as SHA-256 in
magic_link_tokens (15-min TTL, single-use via atomic rowcount UPDATE).
Resend delivers the link; in dev with no API key, EmailService logs a
structured magic_link_dev_fallback event with the URL so the flow works
offline. /admin/auth/consume/{token} verifies, upserts a users row
(display_name from email local-part), creates a sessions row, and drops
an itsdangerous-signed cb_session cookie (HttpOnly, SameSite=Lax, Secure
in prod). /admin renders a placeholder "Welcome, <name>" page pending
Phase 4 CMS. /admin/logout flips revoked_at rather than deleting the row
to preserve the audit trail.

Rate limits use SlowAPI's in-memory limiter (5/15min/IP on login,
20/15min/IP on consume) plus a DB per-email count to catch
IP-rotating abuse. ADMIN_EMAILS enforces allowlist; non-allowlisted
submissions return the same "check your inbox" page with no token
inserted and no email sent (anti-enumeration). Every event lands in
auth_events via AuditService: link_requested, link_consumed,
consume_failed, session_created, session_revoked, rate_limited.

Add a production config validator refusing empty RESEND_API_KEY,
RESEND_FROM, or ADMIN_EMAILS; add PUBLIC_BASE_URL for email link
construction. CSRF deferred to Phase 6 per roadmap scoping; logout
handler marked # TODO(phase-6-csrf).

Mark Phase 3 complete in docs/ROADMAP.md.

.env.example

@@ -43,6 +43,11 @@ FORWARDED_ALLOW_IPS=127.0.0.1
 SESSION_MAX_DAYS=30
 MAGIC_LINK_TTL_MIN=15
 
+# --- Public URL for link construction --------------------------------------
+# Absolute base URL (scheme+host+port) used to build outbound links such as
+# the magic-link auth email. Override for production.
+PUBLIC_BASE_URL=http://127.0.0.1:8000
+
 # --- Build metadata ---------------------------------------------------------
 # Injected at Docker build time. Surfaced by /healthz. Optional in dev.
 GIT_COMMIT_SHA=unknown