Add local auth: users, sessions, register/login/logout/me (#4)

Implements pansy's local (email + password) authentication and the
session layer that OIDC (#5) will also reuse.

- store: users.go (create/get-by-id/get-by-email/count) and sessions.go
  (create/get/touch/delete/delete-expired), scanning the existing 0001
  schema.
- service: the business-logic seam. auth.go (Register/Login/session
  lifecycle/Providers) + password.go (argon2id, 64 MiB/1/4, PHC-encoded,
  constant-time verify) + service.go (Service, clock injection, token
  hashing). First user is admin; closed registration still allows the
  bootstrap user; unknown-email and wrong-password are indistinguishable
  (same error, same argon2 work via a dummy hash).
- api: POST /auth/register|login|logout, GET /auth/me|providers, plus a
  requireAuth middleware that resolves the HttpOnly session cookie
  (SameSite=Lax, Secure under https) to the actor. Handlers stay thin.
- main: wires the service and a periodic expired-session sweep; sessions
  are also dropped lazily on access. Sliding 30-day expiry.
- tests: service (register/login/expiry/renewal/cleanup, password) and
  api (cookie flow, middleware, validation, providers).

Verified end-to-end via curl: register -> me -> restart -> session
persists -> logout -> 401.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

go.mod

@@ -5,6 +5,7 @@ go 1.26.2
 require (
 	github.com/gin-gonic/gin v1.10.1
 	github.com/samber/slog-gin v1.15.0
+	golang.org/x/crypto v0.31.0
 	modernc.org/sqlite v1.34.4
 )
 
@@ -36,7 +37,6 @@ require (
 	go.opentelemetry.io/otel v1.29.0 // indirect
 	go.opentelemetry.io/otel/trace v1.29.0 // indirect
 	golang.org/x/arch v0.8.0 // indirect
-	golang.org/x/crypto v0.31.0 // indirect
 	golang.org/x/net v0.33.0 // indirect
 	golang.org/x/sys v0.28.0 // indirect
 	golang.org/x/text v0.21.0 // indirect