feat(schema): protobuf event registry + buf CI gate (#54)

- Add proto schemas in packages/shared-types/events/ (oo.events.v1):
  envelope.proto, signals.proto, integration.proto
- buf.yaml with STANDARD lint + FILE breaking-change rules
- .gitea/workflows/buf-check.yaml: lint + breaking check on every PR
  touching events/ (needs a Gitea Actions runner to execute)
- scripts/buf-check.sh: local equivalent of the CI check
- NormalizedEvent TS envelope gains eventId, schemaVersion, producer
  to align with the proto Envelope message
- ml/serving/schemas.py: pydantic models mirroring the v1 proto types
- nats_consumer.py: validate payloads via pydantic instead of raw .get()

A field-rename PR will now fail buf breaking with exit code 100 and
show the offending messages. To make a breaking change: keep the old
field reserved, add the new one, bump schema_version to v2.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/nats_consumer.py

@@ -23,6 +23,8 @@ import time
 from pathlib import Path
 from typing import Optional
 
+from schemas import TaskSyncedPayload, TipFeedbackPayload
+
 logger = logging.getLogger(__name__)
 
 NATS_URL = os.getenv("NATS_URL", "")
@@ -48,19 +50,18 @@ def _sync_meta_path(state_dir: Path, user_id: str) -> Path:
 
 async def _handle(subject: str, payload: dict, state_dir: Path) -> None:
     if subject == "signals.task.synced":
-        user_id = payload.get("userId", "")
-        if user_id:
-            p = _sync_meta_path(state_dir, user_id)
-            p.write_text(json.dumps({
-                "last_sync_ts": payload.get("syncedAt") or time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime()),
-                "task_count": payload.get("count", 0),
-            }))
-            logger.info("[nats] task_synced user=%s count=%s", user_id, payload.get("count"))
+        msg = TaskSyncedPayload.model_validate(payload)
+        p = _sync_meta_path(state_dir, msg.userId)
+        p.write_text(json.dumps({
+            "last_sync_ts": msg.syncedAt,
+            "task_count": msg.count,
+        }))
+        logger.info("[nats] task_synced user=%s count=%s", msg.userId, msg.count)
     elif subject == "signals.tip.feedback":
+        msg = TipFeedbackPayload.model_validate(payload)
         logger.info(
             "[nats] tip_feedback user=%s tip=%s action=%s reward=%s",
-            payload.get("userId"), payload.get("tipId"),
-            payload.get("action"), payload.get("reward"),
+            msg.userId, msg.tipId, msg.action, msg.reward,
         )
     else:
         logger.debug("[nats] unhandled subject=%s", subject)