feat(logging): structured JSON logs, uvicorn access log included

Hold Slayer's logs are shipped to Loki by the host's Alloy agent, which
reads container stdout. Text lines arrive there as an opaque blob:
filtering on a status code meant regex over a formatted string. This adds
LOG_FORMAT=json (default "text", so local dev stays readable) rendering
one JSON object per line.

Two parts were less obvious than a format= argument would suggest, and
both are why this is a module rather than a basicConfig tweak:

Uvicorn attaches its own handlers to `uvicorn` and `uvicorn.access` with
propagate=False, so configuring only the root logger would have left the
access log — the highest-volume, most useful stream — as colourised text
next to our JSON. configure_logging clears those handlers and re-enables
propagation, and is called both at import (for startup config checks) and
in lifespan (uvicorn configures itself after importing the app). The
__main__ path passes log_config=None so uvicorn never applies its own.

The access record's payload lives in record.args as a 5-tuple, not in the
message. Formatting it would throw the structure away and force Loki to
parse it back out, so the tuple is unpacked into real fields and
status_code is emitted as a number for range filtering.

Also drops uvicorn's `color_message` extra, an ANSI-coloured duplicate of
the message that generic extra-promotion would otherwise copy into every
startup line — the same unreadable-in-Grafana problem recently fixed for
the lab's Asterisk logs.

Verified against a real uvicorn server: 39/39 lines valid JSON, zero ANSI
escapes, no duplicates, access lines structured with correct status codes;
text mode unchanged. Thread name is included off the main thread, since
"which execution context logged this" is the first question when debugging
across the asyncio/Sippy/PJSUA2 boundary. SecretStr extras stay masked.

README Phase 4 item ticked; LOG_FORMAT and the previously-undocumented
LOG_LEVEL added to the config table and .env.example.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.env.example

@@ -100,6 +100,9 @@ HOST=0.0.0.0
 PORT=8000
 DEBUG=false
 LOG_LEVEL=info
+# Log rendering: "text" (human-readable) or "json" (one object per line, for
+# Loki/Alloy). The Docker image sets json; text is the default for local dev.
+LOG_FORMAT=text
 
 # --- Safety ---
 # Max simultaneous calls the gateway will place (REST + MCP)