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>

.specify/memory/constitution.md

@@ -1,17 +1,15 @@
 <!--
   Sync Impact Report
   ==================
-  Version change: 1.0.0 -> 1.1.0
-  Bump rationale: MINOR. Principle V is materially expanded: the tracing
-    mechanism changed (tracing + tracing-opentelemetry in every build), and
-    it adds two new MUST rules: no manual Context threading, and carrying
-    the parent span across spawns. No principle is removed or redefined in
-    intent.
+  Version change: 1.1.0 -> 1.2.0
+  Bump rationale: MINOR. Principle V's logging rule is materially changed:
+    logs are no longer a separate, build-profile-dependent pipeline; they
+    share the tracing subscriber and export correlated with traces in any
+    build (#147). No principle is removed or redefined in intent.
   Modified principles:
-    - V. Observability: mechanism + rules updated for the tracing migration
-      (map #135, PR #148)
+    - V. Observability: logging bullet rewritten; span events limited to
+      WARN/ERROR
   Also updated:
-    - I. Layered Architecture: Database Layer "traced" wording
     - Technology Stack & Constraints: Tracing line
   Added sections: N/A
   Removed sections: N/A
@@ -22,11 +20,11 @@
     - .specify/templates/checklist-template.md — ✅ no changes needed
     - .specify/templates/agent-file-template.md — ✅ no changes needed
   Runtime guidance:
-    - CLAUDE.md "Tracing" — ✅ already consistent (conventions landed with #148);
-      OTLP env line + dependency list updated alongside this amendment
-    - README.md — ✅ RUST_LOG default/gating corrected; OTLP_OTLS_ENDPOINT added
-    - .env.example — ✅ telemetry header corrected
-  Follow-up TODOs: None
+    - CLAUDE.md "Tracing" — ✅ logs bullet added; dependency list updated
+    - README.md — ✅ RUST_LOG default + OTLP_OTLS_ENDPOINT now cover logs
+    - .env.example — ✅ telemetry header covers logs
+  Follow-up TODOs: migrate `log::` macros to `tracing::` (bridged records
+    export with target "log"; module survives only in code.namespace)
 -->
 
 # ImageApi Constitution
@@ -113,8 +111,11 @@ All database operations and HTTP handlers MUST be instrumented with
   an uncarried spawn orphans its spans onto a fresh trace.
 - `RUST_LOG` gates span export as well as console output (one `EnvFilter`).
   Deployments MUST run at `info` or `debug`.
-- Logs are a separate pipeline: `env_logger` in debug, OTel log export in
-  release.
+- Logs share the tracing subscriber: `log::` records are bridged into
+  `tracing`, so both print in one console format. With
+  `OTLP_OTLS_ENDPOINT` set (any build), events export as OTel log records
+  carrying the enclosing span's trace_id / span_id. Only WARN and ERROR
+  events are also recorded on spans as span events.
 - Prometheus metrics (`imageserver_image_total`,
   `imageserver_video_total`) MUST be maintained for key counters.
 - Errors MUST be logged at `error!` level with sufficient context
@@ -136,8 +137,8 @@ job in one trace by construction.
 - **Image Processing**: `image` crate for thumbnails, `kamadak-exif`
   for EXIF extraction
 - **Tracing**: `tracing` + `tracing-opentelemetry` (every build), OTLP
-  export when `OTLP_OTLS_ENDPOINT` is set; logs via `env_logger` (debug) /
-  OTel log export (release)
+  export when `OTLP_OTLS_ENDPOINT` is set; logs through the same
+  subscriber via `opentelemetry-appender-tracing` (`log::` bridged)
 - **Testing**: `cargo test`, `actix_web::test`, in-memory SQLite
 
 External dependencies (ffmpeg, Ollama) are optional runtime
@@ -177,4 +178,4 @@ MUST comply with these principles.
   these principles. The CLAUDE.md file provides runtime development
   guidance and MUST remain consistent with this constitution.
 
-**Version**: 1.1.0 | **Ratified**: 2026-02-26 | **Last Amended**: 2026-09-29
+**Version**: 1.2.0 | **Ratified**: 2026-02-26 | **Last Amended**: 2026-09-29