feat: distributed tracing and structured logging (#38)

[VECTORG99]

Distributed Tracing & Structured Logging (#38)

All 3 Python services now have structured logging with correlation ID propagation.

Changes

• api/middleware/correlation_id.py (NEW) — Contextvars-based correlation ID propagation + CorrelationLogFilter
• api/middleware/request_id.py — Uses set_correlation_id() from correlation_id module
• api/logging_config.py — Added CorrelationLogFilter to stream and file handlers
• api/main.py — Wired RequestIdMiddleware as outermost middleware
• ai-service/logging_config.py (NEW) — Full structured logging (JSON+Dev formatters, correlation IDs, rotating file handler)
• ai-service/main.py — Added setup_logging() on lifespan, CorrelationMiddleware for X-Request-ID
• worker/logging_config.py — Added ContextVar CID infrastructure, CorrelationLogFilter, request_id field
• worker/watchers/vault_watcher.py — Worker-level CID on startup, X-Request-ID on all HTTP calls
• worker/tasks/joidy_daily_writer.py — Writer-level CID, X-Request-ID on all POST calls
• api/routers/ai.py — Propagates X-Request-ID to ai-service on /classify and /usage

Tracing flow

Worker (generates worker-xxxx CID)
  → X-Request-ID → API (RequestIdMiddleware reads + sets contextvar)
    → log records include request_id field
    → X-Request-ID → AI-service (CorrelationMiddleware reads + sets contextvar)
      → log records include request_id field

Closes #38