feat(db): rebuild Codex usage on upgrade and via maintenance action

Schema v16 wipes codex_session detail rows, _codex_session rollups and
Codex rollout cursors inside the migration savepoint (cursor deletion
uses pure shape matching so CODEX_HOME drift cannot orphan cursors);
the next session sync re-imports history from source JSONL under the
corrected importer. Fresh installs traverse the same branch as a no-op.

Add a manual "Rebuild Codex usage" maintenance action (single-flight,
hard-fail backup before reset, unconditional refresh notification even
when reimport is empty or fails after reset) with a destructive confirm
dialog, result toast and four-locale strings. Historical proxy-side
duplicate rows are intentionally left untouched; history whose source
JSONL was already deleted cannot be reconstructed.

src/lib/api/usage.ts

@@ -180,6 +180,10 @@ export const usageApi = {
     return invoke("sync_session_usage");
   },
 
+  rebuildCodexUsage: async (): Promise<SessionSyncResult> => {
+    return invoke("rebuild_codex_usage");
+  },
+
   getDataSourceBreakdown: async (): Promise<DataSourceSummary[]> => {
     return invoke("get_usage_data_sources");
   },