feat(db): metrics ledger → DB-first unit_metrics table (schema v54)

- Add unit_metrics and project_metrics_meta tables in schema v54
- Export upsertUnitMetrics, getAllUnitMetrics, pruneUnitMetrics,
  getProjectStartedAt, setProjectStartedAt from sf-db.js
- Rewrite metrics.js disk I/O: remove json-persistence/paths imports,
  replace saveJsonFile/loadJsonFile with DB calls
- Public API surface unchanged: loadLedgerFromDisk, getLedger,
  pruneMetricsLedger all return same shapes
- Update schema version assertion in sf-db-migration.test.mjs to 54

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/resources/extensions/sf/metrics.js

@@ -556,7 +556,7 @@ export function pruneMetricsLedger(base, keepCount) {
  * Load ledger from DB without initializing in-memory state.
  * Used by history/export commands outside of autonomous mode.
  */
-export function loadLedgerFromDisk(base) {
+export function loadLedgerFromDisk(_base) {
 	const db = getDatabase();
 	if (!db) return null;
 	try {
@@ -567,7 +567,7 @@ export function loadLedgerFromDisk(base) {
 		return null;
 	}
 }
-function loadLedger(base) {
+function loadLedger(_base) {
 	const db = getDatabase();
 	if (!db) return defaultLedger();
 	try {
@@ -598,7 +598,7 @@ function deduplicateUnits(units) {
 	}
 	return Array.from(map.values());
 }
-function saveLedger(base, data) {
+function saveLedger(_base, data) {
 	const db = getDatabase();
 	if (!db) return;
 	try {