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_test.go

@@ -5,6 +5,15 @@ import (
 	"testing"
 )
 
+// insertTestRelease inserts a minimal external_releases row for use in tests that need FK satisfaction.
+func insertTestRelease(db *DB, rgid, artistID string) error {
+	_, err := db.Conn().Exec(
+		"INSERT OR IGNORE INTO external_releases (rgid, artist_id, title) VALUES (?, ?, ?)",
+		rgid, artistID, "Test Release "+rgid,
+	)
+	return err
+}
+
 // TestMarkNotificationSent_New verifies inserting a new notification record.
 func TestMarkNotificationSent_New(t *testing.T) {
 	db, err := New(":memory:")
@@ -13,6 +22,12 @@ func TestMarkNotificationSent_New(t *testing.T) {
 	}
 	defer db.Close()
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
+	if err := insertTestRelease(db, "rgid-1", "artist-1"); err != nil {
+		t.Fatalf("insertTestRelease: %v", err)
+	}
 	if err := MarkNotificationSent(db, "rgid-1"); err != nil {
 		t.Fatalf("MarkNotificationSent() error: %v", err)
 	}
@@ -34,6 +49,12 @@ func TestMarkNotificationSent_Idempotent(t *testing.T) {
 	}
 	defer db.Close()
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
+	if err := insertTestRelease(db, "rgid-1", "artist-1"); err != nil {
+		t.Fatalf("insertTestRelease: %v", err)
+	}
 	if err := MarkNotificationSent(db, "rgid-1"); err != nil {
 		t.Fatalf("first MarkNotificationSent() error: %v", err)
 	}
@@ -60,6 +81,12 @@ func TestIsNotificationSent_True(t *testing.T) {
 	}
 	defer db.Close()
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
+	if err := insertTestRelease(db, "rgid-1", "artist-1"); err != nil {
+		t.Fatalf("insertTestRelease: %v", err)
+	}
 	if err := MarkNotificationSent(db, "rgid-1"); err != nil {
 		t.Fatalf("MarkNotificationSent() error: %v", err)
 	}
@@ -118,6 +145,12 @@ func TestGetUnnotifiedReleases_AllUnnotified(t *testing.T) {
 		{RGID: "rg3", ArtistID: "artist-2", Title: "Single 1", Type: "single", ReleaseDate: "2024-03-01"},
 	}
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist(artist-1): %v", err)
+	}
+	if err := insertTestArtist(db, "artist-2"); err != nil {
+		t.Fatalf("insertTestArtist(artist-2): %v", err)
+	}
 	for _, r := range releases {
 		if err := SaveExternalRelease(db, &r); err != nil {
 			t.Fatalf("SaveExternalRelease(%s) error: %v", r.RGID, err)
@@ -147,6 +180,12 @@ func TestGetUnnotifiedReleases_SomeNotified(t *testing.T) {
 		{RGID: "rg3", ArtistID: "artist-2", Title: "Single 1", Type: "single", ReleaseDate: "2024-03-01"},
 	}
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist(artist-1): %v", err)
+	}
+	if err := insertTestArtist(db, "artist-2"); err != nil {
+		t.Fatalf("insertTestArtist(artist-2): %v", err)
+	}
 	for _, r := range releases {
 		if err := SaveExternalRelease(db, &r); err != nil {
 			t.Fatalf("SaveExternalRelease(%s) error: %v", r.RGID, err)
@@ -189,6 +228,9 @@ func TestGetUnnotifiedReleases_AllNotified(t *testing.T) {
 		{RGID: "rg2", ArtistID: "artist-1", Title: "Album 2"},
 	}
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist(artist-1): %v", err)
+	}
 	for _, r := range releases {
 		if err := SaveExternalRelease(db, &r); err != nil {
 			t.Fatalf("SaveExternalRelease(%s) error: %v", r.RGID, err)
@@ -236,6 +278,15 @@ func TestMarkNotificationSent_MultipleReleases(t *testing.T) {
 	}
 	defer db.Close()
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
+	for _, rgid := range []string{"rg1", "rg2", "rg3"} {
+		if err := insertTestRelease(db, rgid, "artist-1"); err != nil {
+			t.Fatalf("insertTestRelease(%s): %v", rgid, err)
+		}
+	}
+
 	rgids := []string{"rg1", "rg2", "rg3"}
 	for _, rgid := range rgids {
 		if err := MarkNotificationSent(db, rgid); err != nil {