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>

CLAUDE.md

@@ -593,6 +593,14 @@ an incoming `traceparent`). Conventions:
   never hold `span.enter()` across an `.await`.
 - DB: `otel::trace_db_call`; LLM: `otel::start_llm_request_span` (injects
   `traceparent` downstream).
+- **Logs ride the same subscriber.** `log::` records are bridged into
+  `tracing` (the `tracing-log` feature), so there's one console format, and
+  with `OTLP_OTLS_ENDPOINT` set every event is exported as an OTel log record
+  stamped with its span's trace_id / span_id (`opentelemetry-appender-tracing`)
+  — SigNoz links logs to traces. Prefer `tracing::` macros in new code:
+  bridged records export with target `log` (module only in `code.namespace`).
+  The span layer keeps only WARN/ERROR events as span events (error events
+  set span status); `tests/otel_log_correlation.rs` pins all of this.
 - **`RUST_LOG` gates spans, not just logs.** The subscriber's single
   `EnvFilter` sits in front of both the console and the OTel layer, so a span
   below the filter level is never created: it's missing from the export, it
@@ -956,7 +964,7 @@ side if exposing beyond a trusted network.
 - **image**: Thumbnail generation
 - **walkdir**: Directory traversal
 - **rayon**: Parallel processing
-- **tracing** + **tracing-opentelemetry** + **tracing-actix-web**: spans (exported via `opentelemetry-otlp`)
+- **tracing** + **tracing-opentelemetry** + **tracing-actix-web**: spans (exported via `opentelemetry-otlp`); logs via **opentelemetry-appender-tracing**
 - **bcrypt**: Password hashing
 - **infer**: Magic number file type detection