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>

Cargo.toml

@@ -39,11 +39,11 @@ rayon = "1.5"
 path-absolutize = "3.1"
 log = "0.4"
 env_logger = "0.11.5"
-# default-features = false keeps the `tracing-log` bridge OFF so the console
-# fmt layer prints tracing spans only; `log` records stay on their existing
-# path (env_logger in debug, OTel logs in release) without double-printing.
+# `tracing-log` is ON: `try_init` installs the LogTracer bridge, so `log::`
+# records become tracing events and share the one console format + OTel
+# log export (#147 spike).
 tracing = "0.1"
-tracing-subscriber = { version = "0.3", default-features = false, features = ["std", "registry", "env-filter", "fmt", "ansi"] }
+tracing-subscriber = { version = "0.3", default-features = false, features = ["std", "registry", "env-filter", "fmt", "ansi", "tracing-log"] }
 tracing-opentelemetry = "0.32"
 tracing-actix-web = { version = "0.7", features = ["opentelemetry_0_31"] }
 actix-web-prom = "0.9.0"
@@ -54,8 +54,6 @@ rand = "0.8.5"
 opentelemetry = { version = "0.31.0", features = ["default", "metrics", "tracing"] }
 opentelemetry_sdk = { version = "0.31.0", features = ["default", "rt-tokio-current-thread", "metrics", "testing"] }
 opentelemetry-otlp = { version = "0.31.0", features = ["default", "metrics", "tracing", "grpc-tonic"] }
-opentelemetry-stdout = "0.31.0"
-opentelemetry-appender-log = "0.31.0"
 tempfile = "3.20.0"
 regex = "1.11.1"
 exif = { package = "kamadak-exif", version = "0.6.1" }
@@ -76,8 +74,12 @@ indicatif = "0.17"
 uuid = { version = "1.10", features = ["v4", "serde"] }
 chrono-tz = "0.10"
 tzf-rs = "2.1.2"
+opentelemetry-appender-tracing = { version = "0.31", features = ["experimental_use_tracing_span_context", "experimental_metadata_attributes"] }
 
 # Windows lacks system sqlite3, so re-enable the bundled C build there.
 # Linux/macOS use the system library (faster builds, smaller binary).
 [target.'cfg(windows)'.dependencies]
 libsqlite3-sys = { version = "0.35", features = ["bundled"] }
+
+[dev-dependencies]
+tracing-log = "0.2"