feat: add foundation layer (Go module, config, database, Docker)

Squashed commits from foundation-layer branch:

- Initialize Go module and project skeleton (cmd/naviwatcher/main.go)
- Add configuration management with YAML parsing and validation
- Add database layer with schema migrations (artist_settings, external_releases, notifications_sent)
- Add CRUD operations for artist_settings, external_releases, notifications_sent
- Add Docker setup with multi-stage build and docker-compose
- Verify acceptance criteria (tests, vet, fmt)
- Update README.md with build/run/test instructions
- Fix: filter ignored releases in GetUnnotifiedReleases (spec compliance)
- Fix: add FK constraint on notifications_sent.rgid
- Fix: add config.yaml to .gitignore (security)
- Fix: run Docker container as non-root user
- Fix: pin alpine:3.21 instead of alpine:latest
- Fix: wrap migrations in transactions for atomicity

All 49 tests pass, go vet clean, Docker image builds successfully.

cmd/naviwatcher/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"log"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"naviwatcher/internal/config"
+)
+
+const defaultConfigPath = "config.yaml"
+
+func main() {
+	configPath := flag.String("config", defaultConfigPath, "Path to config file")
+	flag.Parse()
+
+	log.Println("NaviWatcher starting...")
+
+	cfg, err := config.LoadConfig(*configPath)
+	if err != nil {
+		log.Fatalf("Failed to load config: %v", err)
+	}
+
+	log.Printf("Config loaded from %s (server: %s:%d)", *configPath, cfg.Server.Host, cfg.Server.Port)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		sig := <-sigCh
+		log.Printf("Received signal %v, shutting down...", sig)
+		cancel()
+	}()
+
+	if err := run(ctx, cfg); err != nil {
+		log.Fatalf("Application error: %v", err)
+	}
+
+	log.Println("NaviWatcher stopped.")
+}
+
+func run(ctx context.Context, cfg *config.Config) error {
+	// Main application loop — blocks until context is cancelled.
+	// Business logic will be added in future tasks.
+	<-ctx.Done()
+
+	return nil
+}