[refactor](trx-rs): resolve all improvement areas (P1–P3)

P1 — High:
- Merge duplicate APRS/HF-APRS decoder tasks into parameterised inner fn
- Merge duplicate FT8/FT4 decoder tasks into shared ftx inner fn
- Add multi-rig state isolation and command routing tests (listener.rs)
- Add background decode evaluate_bookmark unit tests

P2 — Medium:
- Fix decode-log silent flush errors and rotation failure fallback
- Split api.rs (2,831 LOC) into 7 logical modules (decoder, rig, vchan,
  sse, bookmarks, assets, mod)
- Extract background decode decision cascade into pure evaluate_bookmark()
  function with ChannelAction enum
- Relax actix-web pin from =4.4.1 to 4.4
- Replace VDES magic numbers with named constants

P3 — Low:
- Add doc comments to AisDecoder, VdesDecoder, RdsDecoder
- Add debug_assert on turbo decoder interleaver/deinterleaver lengths
- Add tracing info_span! to all 10 decoder block_in_place calls
- Optimize hot-path string cloning in remote_client spectrum loop

https://claude.ai/code/session_01Y3G65hrfsRRjwyBF2qbBmc
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-client/trx-frontend/trx-frontend-http/Cargo.toml

@@ -16,7 +16,7 @@ tokio = { workspace = true, features = ["full"] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 tracing = { workspace = true }
-actix-web = "=4.4.1"
+actix-web = "4.4"
 actix-ws = "0.3"
 tokio-stream = { version = "0.1", features = ["sync"] }
 futures-util = "0.3"