feat(1e): config PANGOLIN_ prefix + store.Open UTC DSN + go:embed migrations (tsk_zRA6fGU1JuHj)

- internal/config: rewrite Config with PANGOLIN_ prefix fields
  (HTTPAddr, AdminAddr, GRPCAddr, MySQL DSN/fields, RedisAddr,
  AutoMigrate, JWTSecret placeholder, WebhookSecret); Load() replaces
  FromEnv(); missing required MySQL vars return named-field error.

- internal/store/mysql.go: single Open(cfg) entry point; uses
  mysql.ParseDSN to structurally override ParseTime=true, Loc=UTC,
  Collation=utf8mb4_unicode_ci, Params[time_zone]='+00:00'; asserts
  SELECT @@session.time_zone=+00:00 after Ping (startup fatal).

- migrations/embed.go: //go:embed *.sql exposes var FS embed.FS.

- internal/store/migrate.go: MigrateUp/MigrateDown/MigrateVersion
  backed by golang-migrate iofs source + mysql driver; ErrNoChange
  treated as success.

- cmd/migrate/main.go: filled — up/down/version subcommands, reads
  config.Load() + store.Open.

- cmd/server/main.go: startup sequence Load → store.Open (UTC assert)
  → MigrateUp (if PANGOLIN_AUTO_MIGRATE=true) → HTTP listen;
  structured slog output at each step.

- internal/store/mysql_test.go: pure-function unit tests for buildDSN
  (empty-fields case + conflicting params overridden case); both pass.
- internal/store/mysql_integration_test.go: //go:build integration;
  testcontainers mysql:8 — UTC assertion + MigrateUp idempotency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/internal/store/migrate.go

@@ -0,0 +1,92 @@
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+
+	"github.com/golang-migrate/migrate/v4"
+	migratemysql "github.com/golang-migrate/migrate/v4/database/mysql"
+	"github.com/golang-migrate/migrate/v4/source/iofs"
+
+	"github.com/wangjia/pangolin/server/migrations"
+)
+
+// MigrateUp runs all pending up migrations.
+// migrate.ErrNoChange is treated as success (idempotent).
+func MigrateUp(db *sql.DB) error {
+	return runMigration(db, func(m *migrate.Migrate) error {
+		return m.Up()
+	})
+}
+
+// MigrateDown rolls back all applied migrations.
+// migrate.ErrNoChange is treated as success.
+func MigrateDown(db *sql.DB) error {
+	return runMigration(db, func(m *migrate.Migrate) error {
+		return m.Down()
+	})
+}
+
+// MigrateVersion returns the currently applied migration version and whether
+// the schema is in a dirty state. Returns version 0 and no error when no
+// migrations have been applied yet.
+func MigrateVersion(db *sql.DB) (uint, bool, error) {
+	m, cleanup, err := newMigrator(db)
+	if err != nil {
+		return 0, false, err
+	}
+	defer cleanup()
+
+	v, dirty, err := m.Version()
+	if errors.Is(err, migrate.ErrNilVersion) {
+		return 0, false, nil
+	}
+	if err != nil {
+		return 0, false, fmt.Errorf("store.migrate: version: %w", err)
+	}
+	return v, dirty, nil
+}
+
+// runMigration opens a migrator, calls fn, and handles ErrNoChange.
+func runMigration(db *sql.DB, fn func(*migrate.Migrate) error) error {
+	m, cleanup, err := newMigrator(db)
+	if err != nil {
+		return err
+	}
+	defer cleanup()
+
+	if err := fn(m); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+		return fmt.Errorf("store.migrate: %w", err)
+	}
+	return nil
+}
+
+// newMigrator creates a golang-migrate instance backed by the embedded SQL
+// files (iofs source) and the provided *sql.DB (mysql driver instance).
+// The caller must invoke cleanup() to release source and driver resources.
+func newMigrator(db *sql.DB) (*migrate.Migrate, func(), error) {
+	src, err := iofs.New(migrations.FS, ".")
+	if err != nil {
+		return nil, nil, fmt.Errorf("store.migrate: iofs source: %w", err)
+	}
+
+	driver, err := migratemysql.WithInstance(db, &migratemysql.Config{})
+	if err != nil {
+		_ = src.Close()
+		return nil, nil, fmt.Errorf("store.migrate: mysql driver: %w", err)
+	}
+
+	m, err := migrate.NewWithInstance("iofs", src, "mysql", driver)
+	if err != nil {
+		_ = src.Close()
+		_ = driver.Close()
+		return nil, nil, fmt.Errorf("store.migrate: new migrator: %w", err)
+	}
+
+	cleanup := func() {
+		// Close source and driver; errors here are non-fatal cleanup.
+		_, _ = m.Close()
+	}
+	return m, cleanup, nil
+}