fix: zero compiler warnings + readiness endpoint

- Fix all 106 warnings: unused imports, dead struct fields (_prefix),
  dead_code annotations on bin-only items in ingest_worker.rs
- Add GET /ready endpoint (readiness probe, checks DB)
- Keep GET /health lightweight (liveness, no DB check)
- Add ServiceMonitor for Prometheus scraping
- Add tracing-subscriber json feature for structured logs
- 760 tests passing, 0 warnings, 0 errors

Cargo.lock

@@ -3986,6 +3986,16 @@ dependencies = [
  "tracing-core",
 ]
 
+[[package]]
+name = "tracing-serde"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "704b1aeb7be0d0a84fc9828cae51dab5970fee5088f83d1dd7ee6f6246fc6ff1"
+dependencies = [
+ "serde",
+ "tracing-core",
+]
+
 [[package]]
 name = "tracing-subscriber"
 version = "0.3.23"
@@ -3993,11 +4003,14 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cb7f578e5945fb242538965c2d0b04418d38ec25c79d160cd279bf0731c8d319"
 dependencies = [
  "nu-ansi-term",
+ "serde",
+ "serde_json",
  "sharded-slab",
  "smallvec",
  "thread_local",
  "tracing-core",
  "tracing-log",
+ "tracing-serde",
 ]
 
 [[package]]