feat: Phase 5 - Metrics and dashboard API (T-020)

- Add GET /metrics/summary endpoint with global coverage counts and percentage
- Add GET /metrics/by-tactic endpoint with per-tactic coverage breakdown
- Handle multi-tactic techniques (comma-separated) counting in each tactic
- Add CoverageSummary and TacticCoverage Pydantic schemas
- Update README with metrics endpoints and project structure

backend/app/main.py

@@ -9,6 +9,7 @@ from app.routers import techniques as techniques_router
 from app.routers import tests as tests_router
 from app.routers import evidence as evidence_router
 from app.routers import system as system_router
+from app.routers import metrics as metrics_router
 from app.storage import ensure_bucket_exists
 from app.jobs.mitre_sync_job import start_scheduler, scheduler
 
@@ -45,6 +46,7 @@ app.include_router(techniques_router.router, prefix="/api/v1")
 app.include_router(tests_router.router, prefix="/api/v1")
 app.include_router(evidence_router.router, prefix="/api/v1")
 app.include_router(system_router.router, prefix="/api/v1")
+app.include_router(metrics_router.router, prefix="/api/v1")
 
 
 @app.get("/health")