feat(recommender): LLM schema validation + hardcoded fallback tips on AI failure (#90)

Python (ml/serving):
- Validate tip item after JSON parse: non-empty content, valid kind
- Retry on schema failure with a targeted clarification prompt, same 2× retry budget
- JSON parse failures keep the existing retry suffix

TypeScript (recommender):
- Add TipSource 'fallback' to shared-types
- FALLBACK_TIPS: 12 general-purpose life tips (hardcoded, no DB read)
- fetchOrchestratorTip returns {ok} discriminated union instead of null
- On !res.ok or fetch error: serve a random fallback tip with rationale 'AI service issues'
- Update tests: 204 path removed; both failure cases now expect source='fallback'

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

ml/serving/main.py

@@ -293,6 +293,25 @@ _RETRY_SUFFIX_OBJ = (
     "Reply ONLY with the JSON object — no prose, no markdown fences."
 )
 
+_RETRY_SUFFIX_SCHEMA = (
+    "\n\nYour previous response parsed as JSON but was missing required fields. "
+    'Reply ONLY with a JSON object containing "content" (non-empty string) and "kind" '
+    '(one of: advice, task, insight, reminder) — no prose, no markdown fences.'
+)
+
+_VALID_KINDS = {"advice", "task", "insight", "reminder"}
+
+
+def _validate_tip_item(item: dict) -> str | None:
+    """Return an error string if item fails schema, else None."""
+    content = item.get("content", "")
+    if not isinstance(content, str) or not content.strip():
+        return "missing or empty 'content' field"
+    kind = item.get("kind", "")
+    if kind and kind not in _VALID_KINDS:
+        return f"invalid kind '{kind}', must be one of {_VALID_KINDS}"
+    return None
+
 
 @app.post("/agents/{agent_id}/compute", response_model=AgentComputeResponse)
 async def compute_agent(agent_id: str, req: AgentComputeRequest) -> AgentComputeResponse:
@@ -504,11 +523,15 @@ async def recommend(req: RecommendRequest) -> RecommendResponse:
                             text = text[4:]
                     parsed = json.loads(text)
                     item: dict = parsed[0] if isinstance(parsed, list) else parsed
+                    schema_err = _validate_tip_item(item)
+                    if schema_err:
+                        raise ValueError(schema_err)
                     break
                 except (json.JSONDecodeError, ValueError, IndexError) as exc:
                     last_parse_error = str(exc)
                     messages.append({"role": "assistant", "content": last_raw})
-                    messages.append({"role": "user", "content": _RETRY_SUFFIX_OBJ})
+                    is_schema_err = not isinstance(exc, json.JSONDecodeError)
+                    messages.append({"role": "user", "content": _RETRY_SUFFIX_SCHEMA if is_schema_err else _RETRY_SUFFIX_OBJ})
             else:
                 _end_span(llm_span, status="ERROR")
                 _end_span(root, status="ERROR")