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/external_releases_test.go

@@ -5,6 +5,15 @@ import (
 	"testing"
 )
 
+// insertTestArtist inserts a minimal artist_settings row for use in tests that need FK satisfaction.
+func insertTestArtist(db *DB, id string) error {
+	_, err := db.Conn().Exec(
+		"INSERT OR IGNORE INTO artist_settings (id, name) VALUES (?, ?)",
+		id, "Test Artist "+id,
+	)
+	return err
+}
+
 // TestGetExternalRelease_Found verifies retrieving an existing release.
 func TestGetExternalRelease_Found(t *testing.T) {
 	db, err := New(":memory:")
@@ -13,7 +22,9 @@ func TestGetExternalRelease_Found(t *testing.T) {
 	}
 	defer db.Close()
 
-	// Insert a row directly.
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %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,
@@ -78,6 +89,9 @@ func TestSaveExternalRelease_Insert(t *testing.T) {
 		IsIgnored:   false,
 	}
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
 	if err := SaveExternalRelease(db, r); err != nil {
 		t.Fatalf("SaveExternalRelease() error: %v", err)
 	}
@@ -102,6 +116,10 @@ func TestSaveExternalRelease_Update(t *testing.T) {
 	}
 	defer db.Close()
 
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
+
 	// Insert initial row.
 	r1 := &ExternalRelease{
 		RGID:        "rgid-1",
@@ -177,6 +195,12 @@ func TestGetExternalReleasesByArtist_Populated(t *testing.T) {
 		{RGID: "rg3", ArtistID: "artist-b", Title: "Album B1", Type: "single", ReleaseDate: "2024-03-01"},
 	}
 
+	if err := insertTestArtist(db, "artist-a"); err != nil {
+		t.Fatalf("insertTestArtist(artist-a): %v", err)
+	}
+	if err := insertTestArtist(db, "artist-b"); err != nil {
+		t.Fatalf("insertTestArtist(artist-b): %v", err)
+	}
 	for _, r := range releases {
 		if err := SaveExternalRelease(db, &r); err != nil {
 			t.Fatalf("SaveExternalRelease(%s) error: %v", r.RGID, err)
@@ -243,6 +267,12 @@ func TestGetIgnoredReleases_Populated(t *testing.T) {
 		{RGID: "rg3", ArtistID: "a2", Title: "Another Ignored", IsIgnored: true},
 	}
 
+	if err := insertTestArtist(db, "a1"); err != nil {
+		t.Fatalf("insertTestArtist(a1): %v", err)
+	}
+	if err := insertTestArtist(db, "a2"); err != nil {
+		t.Fatalf("insertTestArtist(a2): %v", err)
+	}
 	for _, r := range releases {
 		if err := SaveExternalRelease(db, &r); err != nil {
 			t.Fatalf("SaveExternalRelease(%s) error: %v", r.RGID, err)
@@ -279,6 +309,9 @@ func TestSetReleaseIgnored_SetTrue(t *testing.T) {
 		Title:     "Test Album",
 		IsIgnored: false,
 	}
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
 	if err := SaveExternalRelease(db, r); err != nil {
 		t.Fatalf("SaveExternalRelease() error: %v", err)
 	}
@@ -310,6 +343,9 @@ func TestSetReleaseIgnored_SetFalse(t *testing.T) {
 		Title:     "Test Album",
 		IsIgnored: true,
 	}
+	if err := insertTestArtist(db, "artist-1"); err != nil {
+		t.Fatalf("insertTestArtist: %v", err)
+	}
 	if err := SaveExternalRelease(db, r); err != nil {
 		t.Fatalf("SaveExternalRelease() error: %v", err)
 	}