feat(otel): export logs through the tracing pipeline, correlated with traces (#147)

Logs were a separate pipeline: env_logger in debug (never exported) and
opentelemetry-appender-log in release (exported without trace context,
so SigNoz could not link logs to traces).

- tracing-subscriber `tracing-log` feature: try_init installs LogTracer,
  so log:: records become tracing events: one console format, and
  RUST_LOG per-module directives still apply to them.
- opentelemetry-appender-tracing exports events as OTel log records when
  OTLP_OTLS_ENDPOINT is set (any build), stamped with the enclosing
  span's trace_id/span_id. A per-layer filter drops the exporter's own
  crates.
- The OTel span layer keeps only WARN/ERROR events as span events; the
  info/debug copies duplicated the correlated logs.
- Remove env_logger init in the server, init_logs, and the unused
  opentelemetry-appender-log / opentelemetry-stdout deps.
- tests/otel_log_correlation.rs pins correlation (native + bridged),
  the span-event filter, and per-module filtering of bridged records.
- Constitution 1.2.0 (Principle V logging rule), CLAUDE.md, README,
  .env.example.

Closes #147

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -32,8 +32,9 @@ HLS_TIMEOUT_SECONDS=900
 PLAYLIST_CLEANUP_INTERVAL_SECONDS=86400
 
 # ── Telemetry ────────────────────────────────────────────────────────────
-# Span export over OTLP in any build when set (unset = spans stay local).
-# RUST_LOG also gates span export: keep it at info or debug.
+# Span + log export over OTLP in any build when set (unset = nothing
+# exported). Logs carry trace ids, so SigNoz links them to traces.
+# RUST_LOG also gates export: keep it at info or debug.
 # OTLP_OTLS_ENDPOINT=http://localhost:4317
 
 # ── AI Insights — Ollama (local LLM) ────────────────────────────────────