feat: authentication with local accounts, sessions, CSRF, RBAC, and OIDC PKCE (phase 3)

- argon2id (t=3, m=64MiB, p=2) PHC hashing honoring embedded params
- token-bucket rate limiting (10/15min per IP+email) on login/register
- opaque 32B session tokens in Mongo, 30-day sliding expiry, logout-all
- CSRF double-submit cookie/header on authenticated mutations
- bootstrap admin from env with forced first-login password change
- requireAuth/requireRole middleware with disabled-account enforcement
- OIDC code flow with PKCE: lazy discovery, account linking only on
  verified email, auto-created developer accounts
- unit tests (RBAC matrix, CSRF, password, rate limiter) + integration
  suite covering the full auth matrix incl. an in-test fake IdP

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

go.mod

@@ -2,15 +2,21 @@ module bountyboard
 
 go 1.26
 
-require go.mongodb.org/mongo-driver/v2 v2.6.0
+require (
+	github.com/coreos/go-oidc/v3 v3.18.0
+	go.mongodb.org/mongo-driver/v2 v2.6.0
+	golang.org/x/crypto v0.53.0
+	golang.org/x/oauth2 v0.36.0
+)
 
 require (
+	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
 	github.com/klauspost/compress v1.17.6 // indirect
 	github.com/xdg-go/pbkdf2 v1.0.0 // indirect
 	github.com/xdg-go/scram v1.2.0 // indirect
 	github.com/xdg-go/stringprep v1.0.4 // indirect
 	github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 // indirect
-	golang.org/x/crypto v0.33.0 // indirect
-	golang.org/x/sync v0.11.0 // indirect
-	golang.org/x/text v0.22.0 // indirect
+	golang.org/x/sync v0.21.0 // indirect
+	golang.org/x/sys v0.46.0 // indirect
+	golang.org/x/text v0.38.0 // indirect
 )