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/mysql.go

@@ -0,0 +1,97 @@
+package store
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+
+	// Register the mysql driver with database/sql.
+	_ "github.com/go-sql-driver/mysql"
+	"github.com/go-sql-driver/mysql"
+
+	"github.com/wangjia/pangolin/server/internal/config"
+)
+
+// Open is the single canonical MySQL entry point for the Pangolin server.
+//
+// It:
+//  1. Builds or parses the DSN from cfg.
+//  2. Structurally overrides UTC parameters (ParseTime, Loc, Collation,
+//     time_zone session variable) — no string manipulation.
+//  3. Configures the connection pool.
+//  4. Pings the server.
+//  5. Asserts SELECT @@session.time_zone = "+00:00" (startup fatal if the
+//     server ignores our DSN override — e.g. SQL mode forces a different TZ).
+func Open(cfg config.Config) (*sql.DB, error) {
+	dsn, err := buildDSN(cfg)
+	if err != nil {
+		return nil, fmt.Errorf("store.Open: %w", err)
+	}
+
+	db, err := sql.Open("mysql", dsn)
+	if err != nil {
+		return nil, fmt.Errorf("store.Open: %w", err)
+	}
+
+	// Connection pool.
+	db.SetMaxOpenConns(30)
+	db.SetMaxIdleConns(10)
+	db.SetConnMaxLifetime(5 * time.Minute)
+
+	if err := db.Ping(); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("store.Open: ping: %w", err)
+	}
+
+	// Hard assertion: the session time_zone must be "+00:00".
+	// This catches MySQL servers that ignore client-supplied time_zone params.
+	var tz string
+	if err := db.QueryRow("SELECT @@session.time_zone").Scan(&tz); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("store.Open: query @@session.time_zone: %w", err)
+	}
+	if tz != "+00:00" {
+		_ = db.Close()
+		return nil, fmt.Errorf(
+			"store.Open: session time_zone=%q, want +00:00; UTC DSN override failed",
+			tz,
+		)
+	}
+
+	return db, nil
+}
+
+// buildDSN constructs the final DSN from cfg, structurally overriding UTC
+// and utf8mb4 parameters regardless of what the caller supplied.
+// This is a pure function and is tested independently.
+func buildDSN(cfg config.Config) (string, error) {
+	rawDSN := cfg.MySQLDSN
+	if rawDSN == "" {
+		// Build a minimal DSN from individual fields; ParseDSN will validate it.
+		rawDSN = fmt.Sprintf("%s:%s@tcp(%s:%s)/%s",
+			cfg.MySQLUser,
+			cfg.MySQLPassword,
+			cfg.MySQLHost,
+			cfg.MySQLPort,
+			cfg.MySQLDBName,
+		)
+	}
+
+	dsnCfg, err := mysql.ParseDSN(rawDSN)
+	if err != nil {
+		return "", fmt.Errorf("buildDSN: parse DSN: %w", err)
+	}
+
+	// Force UTC — overwrite any caller-supplied values (structural, not string).
+	dsnCfg.ParseTime = true
+	dsnCfg.Loc = time.UTC
+	// Collation implicitly sets charset to utf8mb4 via SET NAMES.
+	dsnCfg.Collation = "utf8mb4_unicode_ci"
+	if dsnCfg.Params == nil {
+		dsnCfg.Params = make(map[string]string)
+	}
+	// The time_zone connection parameter sends SET time_zone = '+00:00' on connect.
+	dsnCfg.Params["time_zone"] = "'+00:00'"
+
+	return dsnCfg.FormatDSN(), nil
+}