Remove backwards compat code: no installed base pre-1.0

- Simplify bootstrapMigrationsTable to just check table existence and
  apply 000.sql if missing — no legacy DB detection
- Remove ensureBootstrapVersionRecorded (legacy backfill path)
- Remove applyBootstrapMigration (separate helper, now inlined)
- Remove TestBootstrapMigrationsTable_ExistingTableBackwardsCompat
- Simplify 000.sql header comment

internal/database/database_test.go

@@ -222,58 +222,3 @@ func TestBootstrapMigrationsTable_FreshDatabase(t *testing.T) {
 		t.Errorf("expected version 000 to be recorded, got count %d", recorded)
 	}
 }
-
-func TestBootstrapMigrationsTable_ExistingTableBackwardsCompat(t *testing.T) {
-	db := openTestDB(t)
-	ctx := context.Background()
-
-	// Simulate an older database that created the table via inline SQL
-	// (without recording version "000").
-	_, err := db.Exec(`
-		CREATE TABLE schema_migrations (
-			version TEXT PRIMARY KEY,
-			applied_at DATETIME DEFAULT CURRENT_TIMESTAMP
-		)
-	`)
-	if err != nil {
-		t.Fatalf("failed to create legacy table: %v", err)
-	}
-
-	// Insert a fake migration to prove the table already existed.
-	_, err = db.Exec("INSERT INTO schema_migrations (version) VALUES ('001')")
-	if err != nil {
-		t.Fatalf("failed to insert legacy version: %v", err)
-	}
-
-	if err := bootstrapMigrationsTable(ctx, db, nil); err != nil {
-		t.Fatalf("bootstrapMigrationsTable failed: %v", err)
-	}
-
-	// Version "000" must now be recorded.
-	var recorded int
-
-	err = db.QueryRow(
-		"SELECT COUNT(*) FROM schema_migrations WHERE version = '000'",
-	).Scan(&recorded)
-	if err != nil {
-		t.Fatalf("failed to check version: %v", err)
-	}
-
-	if recorded != 1 {
-		t.Errorf("expected version 000 to be recorded for legacy DB, got count %d", recorded)
-	}
-
-	// The existing "001" row must still be there.
-	var legacyCount int
-
-	err = db.QueryRow(
-		"SELECT COUNT(*) FROM schema_migrations WHERE version = '001'",
-	).Scan(&legacyCount)
-	if err != nil {
-		t.Fatalf("failed to check legacy version: %v", err)
-	}
-
-	if legacyCount != 1 {
-		t.Errorf("legacy version 001 row missing after bootstrap")
-	}
-}