Implement hosted Conversion Attestation publication (WP-0006-T06)

migrations/0003_attestations.sql: one row per Phase, no UPDATE/DELETE
grant for trf_app (a published attestation is permanent).

src/target_revenue/attestation.py: publish_attestation() is idempotent
(existing row returned unchanged, never regenerated) and derives
conversion_timestamp from the ledger itself via _find_conversion_prefix(),
which finds the earliest prefix where the fold reaches Outstanding
Target = 0 rather than trusting entries[-1] or wall-clock publish time -
so a later unrelated entry can never change an already-converged Phase's
recorded conversion moment. Raises NotConvertedError rather than
fabricating a conversion. Reuses conversion.generate_attestation()
unchanged. Guarded the psycopg import behind TYPE_CHECKING so the pure
helper stays testable under plain system Python.

service/app.py adds GET /phases/{id}/attestation (unauthenticated,
publish-on-first-observation).

tests/test_attestation.py (3 tests, no Docker/Postgres) proves the
earliest-crossing behavior. 6 new Docker-gated tests in
test_ledger_hosting.py cover pre-conversion 404, the core
legal-technical property that /metrics facts/calculations/forecasts
are identical whether or not /attestation was ever called, one-time
publication, signature verification, and DB-level UPDATE/DELETE
rejection.

tests/test_ledger_hosting.py

@@ -30,6 +30,7 @@ REPO_ROOT = Path(__file__).resolve().parents[1]
 MIGRATIONS = [
     REPO_ROOT / "migrations" / "0001_registries.sql",
     REPO_ROOT / "migrations" / "0002_ledger.sql",
+    REPO_ROOT / "migrations" / "0003_attestations.sql",
 ]
 
 pytestmark = pytest.mark.skipif(
@@ -301,3 +302,103 @@ def test_metrics_endpoint_unauthenticated_and_matches_offline_computation(
 
     assert hosted == offline
     assert hosted["facts"]["cumulative_development_credit"] == 4000
+
+
+def test_attestation_not_available_before_conversion(client, pg_container, registered_phase):
+    phase_id = registered_phase["phase"]["id"]
+    e = _entry(phase_id, "trsl:entry:ledgertestattest0001", "development-credit", 1, "2026-08-01T00:00:00Z")
+    resp = client.post(f"/phases/{phase_id}/ledger", json=e, headers=auth_headers(pg_container["token"]))
+    assert resp.status_code == 201
+
+    metrics_resp = client.get(f"/phases/{phase_id}/metrics")
+    assert metrics_resp.json()["facts"]["is_converted"] is False
+
+    attestation_resp = client.get(f"/phases/{phase_id}/attestation")
+    assert attestation_resp.status_code == 404
+
+
+def test_conversion_is_true_independent_of_attestation_call(client, pg_container):
+    """TSD §3.5 legal-technical rule: conversion is already true from the
+    ledger the instant the fold reaches zero; nothing about calling (or not
+    calling) /attestation may change what /metrics already reports."""
+    manifest = golden_manifest()
+    manifest["phase"]["id"] = manifest["phase"]["id"] + "-attest-conv-" + uuid.uuid4().hex[:6]
+    manifest["phase"]["initial_target"]["amount"] = 1000
+    client.post("/phases", json=manifest, headers=auth_headers(pg_container["token"])).raise_for_status()
+    phase_id = manifest["phase"]["id"]
+
+    e = _entry(phase_id, "trsl:entry:ledgertestattest0002", "development-credit", 1000, "2026-08-01T00:00:00Z")
+    client.post(f"/phases/{phase_id}/ledger", json=e, headers=auth_headers(pg_container["token"])).raise_for_status()
+
+    before = client.get(f"/phases/{phase_id}/metrics").json()
+    assert before["facts"]["is_converted"] is True
+    assert before["facts"]["outstanding_target"] == 0
+
+    # No attestation has been requested yet; conversion is already true.
+    attestation_resp = client.get(f"/phases/{phase_id}/attestation")
+    assert attestation_resp.status_code == 200
+
+    after = client.get(f"/phases/{phase_id}/metrics").json()
+    assert after["facts"]["is_converted"] is True
+    # Publishing the attestation must change nothing about the conversion
+    # facts/calculations/forecasts themselves (only `as_of` legitimately
+    # differs, since /metrics stamps wall-clock time on every call).
+    assert after["facts"] == before["facts"]
+    assert after["calculations"] == before["calculations"]
+    assert after["forecasts"] == before["forecasts"]
+
+
+def test_attestation_published_once_and_idempotent_on_reread(client, pg_container):
+    manifest = golden_manifest()
+    manifest["phase"]["id"] = manifest["phase"]["id"] + "-attest-idem-" + uuid.uuid4().hex[:6]
+    manifest["phase"]["initial_target"]["amount"] = 500
+    client.post("/phases", json=manifest, headers=auth_headers(pg_container["token"])).raise_for_status()
+    phase_id = manifest["phase"]["id"]
+
+    e = _entry(phase_id, "trsl:entry:ledgertestattest0003", "development-credit", 500, "2026-08-01T00:00:00Z")
+    client.post(f"/phases/{phase_id}/ledger", json=e, headers=auth_headers(pg_container["token"])).raise_for_status()
+
+    first = client.get(f"/phases/{phase_id}/attestation")
+    assert first.status_code == 200
+    second = client.get(f"/phases/{phase_id}/attestation")
+    assert second.status_code == 200
+    assert first.json() == second.json()  # same signed record, not regenerated
+
+    from target_revenue import validation
+
+    validation.validate_conversion_attestation(
+        {k: v for k, v in first.json().items() if k != "signature"}
+    )
+    assert first.json()["conversion_timestamp"] == "2026-08-01T00:00:00Z"
+    assert first.json()["ledger_checkpoint"] == "trsl:entry:ledgertestattest0003"
+
+
+def test_attestation_verifiable_with_public_key(client, pg_container):
+    manifest = golden_manifest()
+    manifest["phase"]["id"] = manifest["phase"]["id"] + "-attest-verify-" + uuid.uuid4().hex[:6]
+    manifest["phase"]["initial_target"]["amount"] = 500
+    client.post("/phases", json=manifest, headers=auth_headers(pg_container["token"])).raise_for_status()
+    phase_id = manifest["phase"]["id"]
+
+    e = _entry(phase_id, "trsl:entry:ledgertestattest0004", "development-credit", 500, "2026-08-01T00:00:00Z")
+    client.post(f"/phases/{phase_id}/ledger", json=e, headers=auth_headers(pg_container["token"])).raise_for_status()
+
+    published = client.get(f"/phases/{phase_id}/attestation").json()
+    pk_hex = client.get("/public-key").json()["public_key_hex"]
+
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
+
+    from target_revenue import hashing
+
+    public_key = Ed25519PublicKey.from_public_bytes(bytes.fromhex(pk_hex))
+    assert hashing.verify_record_signature(published, published["signature"], public_key)
+
+
+def test_application_role_cannot_update_or_delete_attestations(pg_container):
+    with psycopg.connect(pg_container["app_dsn"]) as conn:
+        with pytest.raises(psycopg.errors.InsufficientPrivilege):
+            conn.execute("UPDATE attestations SET signature = 'x' WHERE phase_id = 'nonexistent'")
+        conn.rollback()
+        with pytest.raises(psycopg.errors.InsufficientPrivilege):
+            conn.execute("DELETE FROM attestations WHERE phase_id = 'nonexistent'")
+        conn.rollback()