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>

src/main.rs

@@ -111,17 +111,9 @@ fn main() -> std::io::Result<()> {
     let system = actix::System::new();
     system.block_on(async {
         // One `tracing` subscriber in every build: env-driven, so dev prints
-        // spans to the console and release exports them over OTLP. The `log`
-        // pipeline stays separate — env_logger in debug, OTel logs in release.
-        otel::init_tracing();
-        #[cfg(debug_assertions)]
-        {
-            env_logger::init();
-        }
-        #[cfg(not(debug_assertions))]
-        {
-            otel::init_logs();
-        }
+        // spans + logs to the console and OTLP_OTLS_ENDPOINT exports both.
+        // `log::` records are bridged into it.
+        let _logger_provider = otel::init_tracing();
 
         // AppState construction loads (and seeds if needed) the libraries
         // table; we use that list to drive the initial thumbnail sweep.