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>

internal/auth/password.go

@@ -0,0 +1,73 @@
+// Package auth implements authentication primitives: argon2id password
+// hashing, opaque token generation, login rate limiting, bootstrap admin
+// creation, and the OIDC login flow.
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"strings"
+
+	"golang.org/x/crypto/argon2"
+)
+
+// Argon2id parameters fixed by spec §7: t=3, m=64MiB, p=2.
+const (
+	argonTime    = 3
+	argonMemory  = 64 * 1024 // KiB
+	argonThreads = 2
+	argonKeyLen  = 32
+	argonSaltLen = 16
+)
+
+// MinPasswordLen is the minimum accepted password length.
+const MinPasswordLen = 8
+
+// HashPassword returns a PHC-format argon2id hash.
+func HashPassword(password string) (string, error) {
+	salt := make([]byte, argonSaltLen)
+	if _, err := rand.Read(salt); err != nil {
+		return "", fmt.Errorf("salt: %w", err)
+	}
+	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
+	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
+		argon2.Version, argonMemory, argonTime, argonThreads,
+		base64.RawStdEncoding.EncodeToString(salt),
+		base64.RawStdEncoding.EncodeToString(key)), nil
+}
+
+// VerifyPassword checks password against a PHC argon2id hash in constant
+// time. The parameters embedded in the hash are honored so stored hashes
+// survive future parameter changes.
+func VerifyPassword(password, encoded string) (bool, error) {
+	parts := strings.Split(encoded, "$")
+	if len(parts) != 6 || parts[1] != "argon2id" {
+		return false, errors.New("not an argon2id hash")
+	}
+	var version int
+	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
+		return false, fmt.Errorf("hash version: %w", err)
+	}
+	if version != argon2.Version {
+		return false, fmt.Errorf("unsupported argon2 version %d", version)
+	}
+	var m uint32
+	var t uint32
+	var p uint8
+	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &m, &t, &p); err != nil {
+		return false, fmt.Errorf("hash params: %w", err)
+	}
+	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
+	if err != nil {
+		return false, fmt.Errorf("hash salt: %w", err)
+	}
+	want, err := base64.RawStdEncoding.DecodeString(parts[5])
+	if err != nil {
+		return false, fmt.Errorf("hash key: %w", err)
+	}
+	got := argon2.IDKey([]byte(password), salt, t, m, p, uint32(len(want)))
+	return subtle.ConstantTimeCompare(got, want) == 1, nil
+}