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/notifications.go

@@ -7,7 +7,7 @@ import (
 // MarkNotificationSent records that a notification has been sent for the given RGID.
 func MarkNotificationSent(db *DB, rgid string) error {
 	_, err := db.Conn().Exec(
-		"INSERT OR IGNORE INTO notifications_sent (rgid) VALUES (?)",
+		"INSERT OR REPLACE INTO notifications_sent (rgid) VALUES (?)",
 		rgid,
 	)
 	if err != nil {
@@ -60,7 +60,7 @@ func GetUnnotifiedReleases(db *DB) ([]ExternalRelease, error) {
 func GetNotificationSentAt(db *DB, rgid string) (string, error) {
 	var sentAt string
 	err := db.Conn().QueryRow(
-		"SELECT sent_at FROM notifications_sent WHERE rgid = ?", rgid,
+		"SELECT sent_at FROM notifications_sent WHERE rgid = ? ORDER BY sent_at DESC LIMIT 1", rgid,
 	).Scan(&sentAt)
 	if err != nil {
 		return "", err