OpenTelemetry telemetry as stdout JSON lines

[mtauha]

Uses FastAPI 0.142's native OpenTelemetry support with app-local providers whose exporters write one JSON object per line to stdout, so telemetry reaches MotherDuck through the existing log export.

• One server span per request (client address and service version added), FastAPI's error/validation logs linked by trace id. Metrics and per-operation spans off; /health excluded; PSX_TELEMETRY=off disables it.
• Custom spans: cache.historical (HIT/MISS/STALE, memory/redis/none tier, cooldown) and psx.fetch (SDK function, proxied, cache-only hit).
• No headers are recorded; a test pins that X-PSX-Proxy credentials never appear in telemetry.
• Exporter side: https://github.com/psxdata/psx-log-exporter/pull/1

Deploys on the next published release; after that, check that the JSON lines reach the FastAPI Cloud logs intact.

Known issue, not changed here: api/requirements.txt (Docker image) still pins psxdata==1.1.1 while pyproject pins 1.2.0.