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/schemas/metrics.py

@@ -0,0 +1,27 @@
+"""Pydantic schemas for coverage-metrics endpoints."""
+
+from pydantic import BaseModel
+
+
+class CoverageSummary(BaseModel):
+    """Global coverage summary across all MITRE ATT&CK techniques."""
+
+    total_techniques: int
+    validated: int
+    partial: int
+    not_covered: int
+    in_progress: int
+    not_evaluated: int
+    coverage_percentage: float  # (validated + partial) / total * 100
+
+
+class TacticCoverage(BaseModel):
+    """Coverage breakdown for a single tactic."""
+
+    tactic: str
+    total: int
+    validated: int
+    partial: int
+    not_covered: int
+    not_evaluated: int
+    in_progress: int