fix: address code review findings

- Fix notifications_sent PK: changed from (rgid, sent_at) to rgid-only PK
  to prevent duplicate RGID rows across seconds. Use INSERT OR REPLACE
  instead of INSERT OR IGNORE for true idempotency.
- Add foreign key constraints to DDL (artist_id references artist_settings,
  rgid references external_releases) per specification.
- Enable PRAGMA foreign_keys=ON and PRAGMA busy_timeout=5000 for concurrent
  access safety.
- Fix GetNotificationSentAt query: add ORDER BY sent_at DESC LIMIT 1 for
  deterministic results.
- Fix config test: change YAML key from 'chat' to 'chat_id' to match struct
  tag, add ChatID assertion.
- Fix migration tracking test: correct error message from "expected 4" to
  "expected 3".
- Remove dead code in TestLoadConfig_InvalidPort: eliminate unused YAML
  template and remove port 0 case (valid, not invalid).
- Remove unused path parameter from buildConfigWithPort helper.
- Remove pointless 100ms sleep in run() and unused time import.
- Remove tautological TestDefaultConfigPath test.
- Update README.md: Go version 1.21+ to 1.25+, placeholder passwords to
  CHANGE_ME.
- Update config.yaml.example: placeholder passwords to CHANGE_ME.
- Update all database tests to insert parent rows first for FK satisfaction.

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

cmd/naviwatcher/main.go

@@ -7,7 +7,6 @@ import (
 	"os"
 	"os/signal"
 	"syscall"
-	"time"
 
 	"naviwatcher/internal/config"
 )
@@ -51,7 +50,5 @@ func run(ctx context.Context, cfg *config.Config) error {
 	// Business logic will be added in future tasks.
 	<-ctx.Done()
 
-	// Graceful shutdown delay to let goroutines finish.
-	time.Sleep(100 * time.Millisecond)
 	return nil
 }

cmd/naviwatcher/main_test.go

@@ -9,12 +9,6 @@ import (
 	"naviwatcher/internal/config"
 )
 
-func TestDefaultConfigPath(t *testing.T) {
-	if defaultConfigPath != "config.yaml" {
-		t.Errorf("expected default config path 'config.yaml', got %q", defaultConfigPath)
-	}
-}
-
 func TestRun_GracefulShutdown(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()