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

README.md

@@ -57,7 +57,7 @@ All configuration is via environment variables; every value has a default, so `.
 | `PANSY_OIDC_BUTTON_LABEL` | `Sign in with SSO` | Label for the OIDC button on the login page.                      |
 | `PANSY_TRUSTED_PROXIES`   | *(none)*           | Comma-separated proxy CIDRs/IPs to trust for client-IP resolution. |
 
-Auth variables are read at startup now and consumed as the auth features land (issues #4/#5).
+Local email/password auth is live (`POST /api/v1/auth/register`, `/auth/login`, `/auth/logout`, `GET /auth/me`, `GET /auth/providers`); the session is an HttpOnly cookie (`Secure` when `PANSY_BASE_URL` is https). The first account registered becomes admin, and it may register even when `PANSY_REGISTRATION=closed` to bootstrap the instance. OIDC (`PANSY_OIDC_*`) lands in #5.
 
 ## Docker & deployment