G3.2/G3.4: real EPT verification + wire the throttle, reopening agent auth

REOPENS the agent path — but only because possession is now actually proven.

EPT verification (api/app/ept.py): ES256 against Eternitas's published key set
at /.well-known/eternitas-keys. algorithms=["ES256"] makes alg:none and
algorithm confusion unrepresentable rather than merely unlikely; issuer and exp
are enforced by the library; an unknown kid is refused.

Order is deliberate: signature FIRST, trust lookup second. These EPTs live ~365
days and carry rev/tru baked in at issuance, so a year-old "rev: false" proves
nothing — revocation and band still come from a live lookup on every request.

Found while building it: real EPTs put the passport in the "sub" claim. The old
code read "passport"/"sub_passport", which no genuine EPT carries — so real
agents were never recognised and ONLY forged tokens ever authenticated. The
bypass was not just a hole, it was the only thing that worked.

Throttle (api/app/throttle.py): BAND_MULTIPLIER and rate_*_per_day were defined
and read by nothing. Now enforced on repo.create and grant.create, counted
against agent_actions (one source of truth, not a private counter that drifts
from the audit log). Fails CLOSED — a limiter that fails open protects you until
the moment something is wrong. Untrusted band is 403 read-only, not 429, because
"slow down" would be a lie.

Tests: 14 behavioral, signing real ES256 tokens with a locally-generated key so
they exercise the crypto path with no network dependency — genuine tokens
accepted, and alg:none / foreign key / tampered payload / expired / wrong issuer
/ unknown kid / missing claims all refused. 74 green.

Co-Authored-By: Claude (Fable 5) <noreply@anthropic.com>

api/app/routes/repos.py

@@ -26,6 +26,7 @@ from pydantic import BaseModel, Field, field_validator
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 
+from api.app import throttle
 from api.app.auth import ActorType, Caller, get_caller
 from api.app.errors import RepairPointer
 from api.app.models.core import (
@@ -208,6 +209,11 @@ async def create_repo(
             remediation_tool=None,
         )
 
+    # Throttle before any side effect. Checking after would let a rate-limited
+    # agent still create the Gitea repo and only then be told no.
+    async with _sessionmaker(request)() as session:
+        await throttle.enforce(session, settings, caller, "repo.create")
+
     gitea = GiteaClient(settings)
     owner = _owner_login(caller)
     await gitea.ensure_user(owner, f"{owner}@windygit.com")
@@ -234,6 +240,8 @@ async def create_repo(
             ),
         )
         session.add(repo)
+        await session.flush()
+        await throttle.record(session, caller, "repo.create", repo_id=repo.id)
         await session.commit()
         await session.refresh(repo)
 
@@ -337,6 +345,7 @@ async def create_grant(
     """
     settings = request.app.state.settings
     async with _sessionmaker(request)() as session:
+        await throttle.enforce(session, settings, caller, "grant.create")
         repo = await _load_repo(session, repo_id, caller)
         is_owner = (caller.identity_id and repo.identity_id == caller.identity_id) or (
             caller.passport and repo.passport == caller.passport
@@ -364,6 +373,8 @@ async def create_grant(
             expires_at=expires,
         )
         session.add(grant)
+        await session.flush()
+        await throttle.record(session, caller, "grant.create", repo_id=repo.id)
         await session.commit()
         await session.refresh(grant)
         grant_id, role = grant.id, grant.role