Tracing: per-request root span via tracing-actix-web middleware (#139)

Add a conventional per-request OTel server span using the standard
tracing-actix-web middleware (TracingLogger) as the outermost middleware,
so the whole request lives under one root span and an incoming W3C
traceparent is extracted onto it via the global text-map propagator
(installed in #138).

extract_context_from_request now sources the operation span's parent from
that root span (tracing::Span::current().context()) instead of re-parsing
the header, so per-handler operation spans nest under the root span as
one trace tree rather than duplicating it as a sibling. Header parsing
remains as a fallback for handlers invoked without the middleware (tests).

The literal per-handler span boilerplate is intentionally NOT removed here;
it becomes #[instrument] in the LLM (#140) and DAO (#141) migrations, where
the &Context threading disappears entirely.

tests/otel_traceparent.rs pins the behavior end-to-end: an incoming
traceparent's trace_id propagates to the request, the root span is parented
on the caller's (remote) span, and the operation span nests under the root
span rather than alongside it.

Cargo.lock

@@ -2124,6 +2124,7 @@ dependencies = [
  "tokio",
  "tokio-util",
  "tracing",
+ "tracing-actix-web",
  "tracing-opentelemetry",
  "tracing-subscriber",
  "tzf-rs",
@@ -2543,6 +2544,12 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "13d2233c9842d08cfe13f9eac96e207ca6a2ea10b80259ebe8ad0268be27d2af"
 
+[[package]]
+name = "mutually_exclusive_features"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e94e1e6445d314f972ff7395df2de295fe51b71821694f0b0e1e79c4f12c8577"
+
 [[package]]
 name = "native-tls"
 version = "0.2.18"
@@ -4359,6 +4366,21 @@ dependencies = [
  "tracing-core",
 ]
 
+[[package]]
+name = "tracing-actix-web"
+version = "0.7.25"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3133a9a17a910bb708dcdcf1ff89d3dd98d412938b5e017c6e42de11b63ef164"
+dependencies = [
+ "actix-web",
+ "mutually_exclusive_features",
+ "opentelemetry",
+ "pin-project-lite",
+ "tracing",
+ "tracing-opentelemetry",
+ "uuid",
+]
+
 [[package]]
 name = "tracing-attributes"
 version = "0.1.31"