T-37: add aggregated historical OMP stats via omp-stats

[jaesbit]

Adds a filterable historical stats dashboard backed by @oh-my-pi/omp-stats.

Changes

• GET /api/usage/stats — new endpoint, filterable by workspace (cwd), model, agentType, and range (1h|24h|7d|30d|90d|all).
• @oh-my-pi/omp-stats integration: syncAllSessions() fires single-flight background sync (default worker pool); getDashboardStats() ensures initDb() on first request. A read-only SQLite handle runs combined WHERE-clause queries for cross-dimensional filtering.
• Session drill-down links: resolved server-side via bridge.listSessions() path map; ancestor walk handles subagent/advisor transcripts at any depth. No raw filesystem paths sent to client.
• source: "sessions" explicit in wire format; routine_runs excluded (satisfies acceptance criterion).
• New protocol types: OmpStatsRange, SessionDrillDownLink, HistoricalModelStats, HistoricalWorkspaceStats, HistoricalAgentTypeStats, AggregatedStatsResponse.
• AggregatedStatsSection in SubscriptionLimitsView: range picker, workspace/model/agentType dropdowns, byModel table with expandable session link chips, byWorkspace and byAgentType tables.

Tests

• 17 unit tests (computeCutoff, resolveParentSessionPath, resolveDeckSession including deep-nested advisor case).
• 7 new route-level tests verifying query-param parsing, bridge forwarding, response shape, source label.
• tsc server: 14 pre-existing errors only. tsc web: clean. Vite build: clean.

Session: auto-work T-37