fix(security): enforce globally-unique WebAuthn credential_id

The webauthn_credentials primary key is (user_id, credential_id), which does
not stop the same credential_id from existing under two users. Usernameless
authentication looks up the credential by id alone, so a duplicate could
resolve to the wrong account. Add a unique index on credential_id (migration
003); duplicate registration now raises DuplicateError.

Refs: porchlight-as2

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_store/test_migrations.py

@@ -13,7 +13,7 @@ async def test_run_migrations_applies_initial() -> None:
     async with aiosqlite.connect(":memory:") as db:
         await db.execute("PRAGMA foreign_keys=ON")
         count = await run_migrations(db, MIGRATIONS_DIR)
-        assert count == 2
+        assert count == 3
         async with db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='users'") as cursor:
             row = await cursor.fetchone()
         assert row is not None
@@ -24,7 +24,7 @@ async def test_run_migrations_skips_already_applied() -> None:
         await db.execute("PRAGMA foreign_keys=ON")
         first_count = await run_migrations(db, MIGRATIONS_DIR)
         second_count = await run_migrations(db, MIGRATIONS_DIR)
-        assert first_count == 2
+        assert first_count == 3
         assert second_count == 0
 
 

tests/test_store/test_sqlite_credential_repo.py

@@ -227,3 +227,21 @@ async def test_delete_webauthn_if_not_last_allows_when_others_exist(
 
     assert await credential_repo.delete_webauthn_if_not_last(alice.userid, b"\x01") is True
     assert len(await credential_repo.get_webauthn_by_user(alice.userid)) == 0
+
+
+async def test_webauthn_credential_id_is_globally_unique(
+    credential_repo: SQLiteCredentialRepository, user_repo: SQLiteUserRepository
+) -> None:
+    # The same credential_id must not be registrable for two different users,
+    # otherwise lookup-by-credential-id could resolve to the wrong account.
+    await user_repo.create(User(userid="u-alice", username="alice2"))
+    await user_repo.create(User(userid="u-bob", username="bob2"))
+    cred_id = b"\xde\xad\xbe\xef"
+
+    await credential_repo.create_webauthn(
+        WebAuthnCredential(user_id="u-alice", credential_id=cred_id, public_key=b"\x01")
+    )
+    with pytest.raises(DuplicateError):
+        await credential_repo.create_webauthn(
+            WebAuthnCredential(user_id="u-bob", credential_id=cred_id, public_key=b"\x02")
+        )