Implement public metrics service (WP-0006-T05)

src/target_revenue/metrics.py: compute_metrics(manifest, entries, as_of)
is pure/deterministic like fold.py, reusing fold.py/conversion.py
unchanged. Returns facts/calculations/forecasts as three explicitly
separated blocks (TrustServicePRD TS-FR-5), covering the mandatory Q9
set plus the recommended velocity/forecast tier - forecasts are always
null rather than populated once a Phase has converted or velocity is
non-positive, so nothing disguises a projection as a fact.

Adds GET /phases/{id}/metrics (unauthenticated per FR-9/FR-10).
tests/test_metrics.py (6 tests) needs no Docker/Postgres and runs
under plain system Python. One new Docker-gated test in
test_ledger_hosting.py proves the hosted /metrics response exactly
matches compute_metrics() run offline against the same export.

src/target_revenue/service/app.py

@@ -18,7 +18,7 @@ from fastapi import Depends, FastAPI, HTTPException, Request
 from psycopg import Connection
 from psycopg_pool import ConnectionPool
 
-from .. import ledger, registry
+from .. import ledger, metrics, registry
 from . import keys
 
 app = FastAPI(title="Target Revenue Trust Service — Registries", version="0.1.0")
@@ -140,3 +140,16 @@ def read_ledger(
     if registry.get_phase_manifest(conn, phase_id) is None:
         raise HTTPException(status_code=404, detail="phase not found")
     return ledger.get_ledger(conn, phase_id)
+
+
+@app.get("/phases/{phase_id}/metrics")
+def read_metrics(
+    phase_id: str, conn: Connection = Depends(get_connection)
+) -> dict[str, Any]:
+    """Public, unauthenticated per FR-9/FR-10 — see `target_revenue.metrics`
+    for the fact/calculation/forecast labeling this response preserves."""
+    manifest = registry.get_phase_manifest(conn, phase_id)
+    if manifest is None:
+        raise HTTPException(status_code=404, detail="phase not found")
+    entries = ledger.get_ledger(conn, phase_id)
+    return metrics.compute_metrics(manifest, entries, metrics.utcnow())