Otel: unify tracing subscriber across dev+release; env-gated OTLP export (#138)

Install ONE `tracing` subscriber in both build profiles via
`otel::init_tracing()` (previously release-only, so spans were a no-op in
dev). Behaviour is now environment-driven, not build-profile-driven:

- OTLP span export happens only when `OTLP_OTLS_ENDPOINT` is set
  (release / staging). Unset -> spans are still created with valid
  contexts (so outgoing `traceparent` injection keeps working) but export
  nowhere; the console `fmt` layer is what shows them.
- `tracing_subscriber::registry()` + `EnvFilter` (RUST_LOG) +
  `OpenTelemetryLayer` + console `fmt` layer, installed in both builds.
- Set the global W3C `TraceContextPropagator`: required by the
  `tracing-actix-web` request middleware (#139) and by the outgoing
  traceparent helpers, both of which read the *global* propagator.
- `tracing-log` bridge stays OFF (`default-features = false`) so the console
  fmt layer prints tracing spans only; `log` records keep their existing
  path (env_logger in debug, OTel logs in release) without double-printing.

Deps: + tracing, tracing-subscriber (env-filter/fmt/ansi), tracing-opentelemetry
0.32 (pins opentelemetry 0.31 + otel_sdk 0.31, no version bump); opentelemetry_sdk
gains the `testing` feature.

Add `tests/otel_propagation.rs`: the regression matrix from spike #133, pinning
the invariant the whole migration rests on -- an OTel context does NOT cross a
bare `tokio::spawn` on a multi-thread runtime (S2/S4 orphan onto a fresh trace)
but DOES with an explicit parent carry across the spawn (S6 `.instrument()`,
S7 `span.enter()`). This is why every spawn in the codebase must carry the parent.

clippy --all-targets clean; 690 lib + 775 integration + regression + 4 doc-tests pass.

Cargo.toml

@@ -39,13 +39,19 @@ 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 = "0.1"
+tracing-subscriber = { version = "0.3", default-features = false, features = ["std", "registry", "env-filter", "fmt", "ansi"] }
+tracing-opentelemetry = "0.32"
 actix-web-prom = "0.9.0"
 prometheus = "0.13"
 lazy_static = "1.5"
 anyhow = "1.0"
 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"] }
+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"