feat(docker-watcher): phase 1 - project scaffold & SQLite store

Initialize Go module, directory structure, and full SQLite store layer:
- 7-table schema (projects, stages, registries, settings, instances,
  deploys, deploy_logs) with auto-migration
- CRUD operations for all entities with proper error handling
- ErrNotFound sentinel for distinguishing 404 from 500 in handlers
- WAL mode, foreign keys, busy timeout pragmas

cmd/server/main.go

@@ -0,0 +1,37 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/alexei/docker-watcher/internal/store"
+)
+
+func main() {
+	dataDir := envOrDefault("DATA_DIR", "./data")
+
+	if err := os.MkdirAll(dataDir, 0o755); err != nil {
+		log.Fatalf("create data directory: %v", err)
+	}
+
+	dbPath := filepath.Join(dataDir, "docker-watcher.db")
+	db, err := store.New(dbPath)
+	if err != nil {
+		log.Fatalf("open store: %v", err)
+	}
+	defer db.Close()
+
+	fmt.Printf("Docker Watcher started. Database: %s\n", dbPath)
+
+	// Future phases will wire up the HTTP server, deployer, poller, etc.
+}
+
+// envOrDefault reads an environment variable or returns the fallback value.
+func envOrDefault(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}