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/schemas.py

@@ -0,0 +1,50 @@
+"""
+Pydantic models mirroring oo.events.v1 proto schemas.
+
+Field names use camelCase to match the proto3 JSON mapping convention
+and the TypeScript payload shapes published by services/api.
+
+Keep in sync with packages/shared-types/events/oo/events/v1/.
+"""
+from __future__ import annotations
+
+from typing import Literal, Optional
+from pydantic import BaseModel
+
+
+class TaskSyncedPayload(BaseModel):
+    userId: str
+    source: str
+    count: int
+    syncedAt: str
+
+
+class TipServedPayload(BaseModel):
+    userId: str
+    tipId: str
+    policy: str
+    servedAt: str
+
+
+class TipFeedbackPayload(BaseModel):
+    userId: str
+    tipId: str
+    action: Literal['done', 'dismiss', 'snooze', 'helpful', 'not_helpful']
+    reward: float
+    dwellMs: Optional[int] = None
+    createdAt: str
+
+
+class TipRewardFailedPayload(BaseModel):
+    userId: str
+    tipId: str
+    reward: float
+    attempts: int
+    error: str
+    failedAt: str
+
+
+class IntegrationTokenExpiredPayload(BaseModel):
+    userId: str
+    provider: str
+    detectedAt: str