Implement Phase 2: password auth (Argon2id + bearer tokens)

- Argon2id password hashing and verification with configurable params
- Bearer token generation (32-byte random), SHA-256 hashed storage,
  TTL-based expiry
- User creation and authentication helpers
- auth_tokens table added to migrations
- 6 tests: hash/verify, wrong password, create/auth user, token
  create/validate, token expiry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/auth/argon2.go

@@ -0,0 +1,77 @@
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"fmt"
+	"strings"
+
+	"golang.org/x/crypto/argon2"
+)
+
+type Argon2Params struct {
+	Memory  uint32
+	Time    uint32
+	Threads uint8
+}
+
+var DefaultParams = Argon2Params{
+	Memory:  65536,
+	Time:    3,
+	Threads: 4,
+}
+
+const saltLen = 16
+const keyLen = 32
+
+// HashPassword generates an Argon2id hash string.
+func HashPassword(password string, params Argon2Params) (string, error) {
+	salt := make([]byte, saltLen)
+	if _, err := rand.Read(salt); err != nil {
+		return "", fmt.Errorf("generate salt: %w", err)
+	}
+
+	key := argon2.IDKey([]byte(password), salt, params.Time, params.Memory, params.Threads, keyLen)
+
+	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
+		argon2.Version,
+		params.Memory, params.Time, params.Threads,
+		base64.RawStdEncoding.EncodeToString(salt),
+		base64.RawStdEncoding.EncodeToString(key),
+	), nil
+}
+
+// VerifyPassword checks a password against an Argon2id hash string.
+func VerifyPassword(password, hash string) (bool, error) {
+	parts := strings.Split(hash, "$")
+	if len(parts) != 6 || parts[1] != "argon2id" {
+		return false, fmt.Errorf("invalid hash format")
+	}
+
+	var v int
+	if _, err := fmt.Sscanf(parts[2], "v=%d", &v); err != nil {
+		return false, fmt.Errorf("parse version: %w", err)
+	}
+
+	var memory uint32
+	var time uint32
+	var threads uint8
+	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time, &threads); err != nil {
+		return false, fmt.Errorf("parse params: %w", err)
+	}
+
+	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
+	if err != nil {
+		return false, fmt.Errorf("decode salt: %w", err)
+	}
+
+	expectedKey, err := base64.RawStdEncoding.DecodeString(parts[5])
+	if err != nil {
+		return false, fmt.Errorf("decode key: %w", err)
+	}
+
+	key := argon2.IDKey([]byte(password), salt, time, memory, threads, uint32(len(expectedKey)))
+
+	return subtle.ConstantTimeCompare(key, expectedKey) == 1, nil
+}