config + store: remove dead knobs and stale schema

Three drift items surfaced in review, each dead on arrival and each
worth trusting a little more at v0.1.0.

config: drop MetricsPollInterval. The field was parsed from TOML
(metrics_poll_interval), stored on DaemonConfig, and ignored by every
consumer — only StatsPollInterval drives the background poll loop.
Users setting it in config.toml saw zero effect. Removed from the TOML
surface, the model constant, and the config test.

daemon: delete ensureDefaultImage. No callers, body was `_ = ctx;
return nil`. Dead since whatever flow used to call it got removed.

store: drop packages_path from the images table. The column was
carried by the baseline migration but never referenced by UpsertImage
(no INSERT / UPDATE mention) or any Go model field — a ghost from a
build pipeline that no longer exists. Added migration id=2
(drop_dead_image_columns) with an idempotent dropColumnIfExists
helper: fresh installs run baseline (creates the column) + 2 (drops
it); legacy DBs where the column was never added get a no-op. Updated
the direct-INSERT SQL in TestGetImageRejectsMalformedTimestamp to
drop the column reference, and added a migration test covering both
install paths (fresh + legacy).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/store/migrations_test.go

@@ -141,6 +141,80 @@ func TestApplyMigrationRollsBackOnBodyError(t *testing.T) {
 	}
 }
 
+// TestMigrateDropDeadImageColumns_AcrossInstallPaths verifies the
+// drop-column migration is correct on both paths it can land on:
+// a fresh install (baseline created the column, migration 2 drops
+// it) and a legacy DB that somehow lost or never had the column
+// (migration 2 is a no-op). Runs migrations end-to-end so the
+// invariant-check is the real system, not the helper in isolation.
+func TestMigrateDropDeadImageColumns_AcrossInstallPaths(t *testing.T) {
+	hasColumn := func(t *testing.T, db *sql.DB, table, column string) bool {
+		t.Helper()
+		rows, err := db.Query("PRAGMA table_info(" + table + ")")
+		if err != nil {
+			t.Fatalf("PRAGMA table_info: %v", err)
+		}
+		defer rows.Close()
+		for rows.Next() {
+			var (
+				cid       int
+				name      string
+				valueType string
+				notNull   int
+				defaultV  sql.NullString
+				pk        int
+			)
+			if err := rows.Scan(&cid, &name, &valueType, &notNull, &defaultV, &pk); err != nil {
+				t.Fatalf("scan table_info row: %v", err)
+			}
+			if name == column {
+				return true
+			}
+		}
+		if err := rows.Err(); err != nil {
+			t.Fatalf("rows.Err: %v", err)
+		}
+		return false
+	}
+
+	t.Run("fresh install drops packages_path", func(t *testing.T) {
+		db := openRawDB(t)
+		if err := runMigrations(db); err != nil {
+			t.Fatalf("runMigrations: %v", err)
+		}
+		if hasColumn(t, db, "images", "packages_path") {
+			t.Fatal("packages_path column survived migration 2 on fresh install")
+		}
+	})
+
+	t.Run("legacy DB without column is a no-op", func(t *testing.T) {
+		db := openRawDB(t)
+		// Simulate a DB whose baseline was applied against a modified
+		// schema that never had packages_path: seed schema_migrations,
+		// run baseline, drop the column out-of-band, then run
+		// runMigrations and expect migration 2 to succeed regardless.
+		if _, err := db.Exec(`CREATE TABLE IF NOT EXISTS schema_migrations (
+			id INTEGER PRIMARY KEY,
+			name TEXT NOT NULL,
+			applied_at TEXT NOT NULL
+		)`); err != nil {
+			t.Fatalf("seed schema_migrations: %v", err)
+		}
+		if err := applyMigration(db, migrations[0]); err != nil {
+			t.Fatalf("apply baseline: %v", err)
+		}
+		if _, err := db.Exec("ALTER TABLE images DROP COLUMN packages_path"); err != nil {
+			t.Fatalf("pre-drop packages_path: %v", err)
+		}
+		if err := runMigrations(db); err != nil {
+			t.Fatalf("runMigrations after manual pre-drop: %v", err)
+		}
+		if hasColumn(t, db, "images", "packages_path") {
+			t.Fatal("packages_path reappeared after runMigrations")
+		}
+	})
+}
+
 func TestRunMigrationsRejectsDuplicateID(t *testing.T) {
 	db := openRawDB(t)
 	orig := migrations