feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

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

server/internal/store/store.go

@@ -0,0 +1,44 @@
+package store
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/glebarez/sqlite"
+	"gorm.io/driver/postgres"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+)
+
+func Open(driver, dsn string, debug bool) (*gorm.DB, error) {
+	cfg := &gorm.Config{}
+	if debug {
+		cfg.Logger = logger.Default.LogMode(logger.Info)
+	} else {
+		cfg.Logger = logger.Default.LogMode(logger.Warn)
+	}
+
+	var dialector gorm.Dialector
+	switch driver {
+	case "postgres":
+		dialector = postgres.Open(dsn)
+	default:
+		if err := os.MkdirAll(filepath.Dir(dsn), 0o755); err != nil {
+			return nil, fmt.Errorf("create data dir: %w", err)
+		}
+		dialector = sqlite.Open(dsn)
+	}
+
+	db, err := gorm.Open(dialector, cfg)
+	if err != nil {
+		return nil, err
+	}
+	sqlDB, err := db.DB()
+	if err != nil {
+		return nil, err
+	}
+	sqlDB.SetMaxOpenConns(20)
+	sqlDB.SetMaxIdleConns(5)
+	return db, nil
+}