feat: JSON schema + validator for scene description training data (#647)

- schema.json: JSON Schema for scene description entries (song, artist,
  beat, timestamp, duration_seconds, lyric_line, scene with
  mood/colors/composition/camera/description)
- validate.py: CLI validator for JSONL files (no jsonschema dependency,
  manual fallback). Validates all fields, types, ranges, empty values.
- .gitea/workflows/validate-training-data.yml: CI check on PR

Tested: 100/100 entries in scene-descriptions-r&b-soul.jsonl pass.

Closes #647

.gitea/workflows/validate-training-data.yml

@@ -0,0 +1,24 @@
+name: Validate Training Data
+
+on:
+  pull_request:
+    paths:
+      - 'training/data/**/*.jsonl'
+      - 'training/data/**/schema.json'
+      - 'training/data/**/validate.py'
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Validate scene description JSONL
+        run: |
+          cd training/data/scene-descriptions
+          python3 validate.py *.jsonl