feat(observability): structured logs, W3C trace IDs, Sentry hooks (#18)

- TS: pino + pino-http; every HTTP request log includes traceId from
  W3C traceparent header (generated if absent); forwarded to ml/serving
  on all /score, /generate, /reward, and /api/ml proxy calls
- Python: structlog JSON; FastAPI middleware binds trace_id via
  contextvars so every log line within a request carries it
- Sentry: optional SENTRY_DSN init in both runtimes (no-op if unset)
- Replace all console.* calls across services/api with pino logger
- Update tests to spy on logger instead of console

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

services/api/src/signals/todoist.ts

@@ -3,6 +3,7 @@ import { db } from '../db/index.js';
 import { integrationTokens } from '../db/schema.js';
 import { eq, and } from 'drizzle-orm';
 import { bus } from '../events/bus.js';
+import { logger } from '../logger.js';
 
 const CACHE_TTL_MS = 30_000;
 
@@ -46,7 +47,7 @@ export class TodoistSignalSource implements SignalSource {
 
     if (!res.ok) {
       if (res.status === 401) {
-        console.error(`[todoist] token expired for user ${userId}`);
+        logger.warn({ userId }, 'todoist: token expired');
         bus.publish('signals.integration.token_expired', {
           userId,
           provider: 'todoist',