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>

.gitea/workflows/buf-check.yaml

@@ -0,0 +1,37 @@
+name: buf-check
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'packages/shared-types/events/**'
+  pull_request:
+    paths:
+      - 'packages/shared-types/events/**'
+
+jobs:
+  buf:
+    name: Lint & breaking-change check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install buf
+        run: |
+          BUF_VERSION=1.50.0
+          curl -sSfL \
+            "https://github.com/bufbuild/buf/releases/download/v${BUF_VERSION}/buf-Linux-x86_64" \
+            -o /usr/local/bin/buf
+          chmod +x /usr/local/bin/buf
+          buf --version
+
+      - name: buf lint
+        run: buf lint packages/shared-types/events
+
+      - name: buf breaking
+        if: github.event_name == 'pull_request'
+        run: |
+          buf breaking packages/shared-types/events \
+            --against ".git#branch=${{ github.base_ref }},subdir=packages/shared-types/events"