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>

internal/database/database_test.go

@@ -119,6 +119,11 @@ func TestExternalReleasesSchema(t *testing.T) {
 	}
 	defer db.Close()
 
+	// Insert parent artist first (FK requirement).
+	_, err = db.Conn().Exec("INSERT INTO artist_settings (id, name) VALUES (?, ?)", "artist-1", "Test Artist")
+	if err != nil {
+		t.Fatalf("insert artist: %v", err)
+	}
 	_, err = db.Conn().Exec(
 		"INSERT INTO external_releases (rgid, artist_id, title, type, release_date, is_ignored) VALUES (?, ?, ?, ?, ?, ?)",
 		"rgid-1", "artist-1", "Test Album", "album", "2024-01-01", false,
@@ -150,6 +155,15 @@ func TestNotificationsSentSchema(t *testing.T) {
 	}
 	defer db.Close()
 
+	// Insert parent artist and release first (FK requirements).
+	_, err = db.Conn().Exec("INSERT INTO artist_settings (id, name) VALUES (?, ?)", "artist-1", "Test Artist")
+	if err != nil {
+		t.Fatalf("insert artist: %v", err)
+	}
+	_, err = db.Conn().Exec("INSERT INTO external_releases (rgid, artist_id, title) VALUES (?, ?, ?)", "rgid-1", "artist-1", "Test Album")
+	if err != nil {
+		t.Fatalf("insert release: %v", err)
+	}
 	_, err = db.Conn().Exec(
 		"INSERT INTO notifications_sent (rgid) VALUES (?)",
 		"rgid-1",
@@ -192,6 +206,6 @@ func TestMigrationTracking(t *testing.T) {
 
 	// We have 3 recorded migrations: artist_settings, external_releases, notifications_sent.
 	if count != 3 {
-		t.Errorf("expected 4 applied migrations, got %d", count)
+		t.Errorf("expected 3 applied migrations, got %d", count)
 	}
 }