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.go

@@ -26,6 +26,18 @@ func New(dbPath string) (*DB, error) {
 		return nil, fmt.Errorf("set WAL mode: %w", err)
 	}
 
+	// Enable foreign key enforcement.
+	if _, err := conn.Exec("PRAGMA foreign_keys=ON"); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("enable foreign keys: %w", err)
+	}
+
+	// Set busy timeout to handle concurrent write contention.
+	if _, err := conn.Exec("PRAGMA busy_timeout=5000"); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("set busy timeout: %w", err)
+	}
+
 	db := &DB{conn: conn}
 	if err := db.migrate(); err != nil {
 		conn.Close()
@@ -74,7 +86,7 @@ func (db *DB) migrate() error {
 			name: "002_create_external_releases",
 			sql: `CREATE TABLE IF NOT EXISTS external_releases (
 				rgid TEXT PRIMARY KEY,
-				artist_id TEXT NOT NULL,
+				artist_id TEXT NOT NULL REFERENCES artist_settings(id),
 				title TEXT NOT NULL,
 				type TEXT,
 				release_date TEXT,
@@ -84,9 +96,8 @@ func (db *DB) migrate() error {
 		{
 			name: "003_create_notifications_sent",
 			sql: `CREATE TABLE IF NOT EXISTS notifications_sent (
-				rgid TEXT NOT NULL,
-				sent_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-				PRIMARY KEY (rgid, sent_at)
+				rgid TEXT PRIMARY KEY REFERENCES external_releases(rgid),
+				sent_at DATETIME DEFAULT CURRENT_TIMESTAMP
 			);`,
 		},
 	}