refactor: decompose large functions and remove dead code

- Decompose modify_request() into 7 single-responsibility helpers
- Decompose handle_http_over_tls(): extract read_full_request, dispatch_stream_events
- Promote connect_upstream/resolve_upstream to module-level functions
- Split standalone.rs (1238 lines) into 4 submodules:
  standalone/mod.rs, spawn.rs, discovery.rs, stub.rs
- Extract proto wire primitives into proto/wire.rs
- Remove 6 dead MitmStore methods
- Remove dead SessionResult, DEFAULT_SESSION, get_or_create
- Remove dead decode_varint_at, extract_conversation_id
- Clean all unused imports across 10 files
- Suppress structural dead_code warnings on deserialization fields

Warnings: 20 -> 0. All 43 tests pass.

src/api/util.rs

@@ -122,10 +122,17 @@ pub(crate) fn now_unix() -> u64 {
         .as_secs()
 }
 
+/// Default request timeout in seconds (used by serde defaults).
+pub(crate) fn default_timeout() -> u64 {
+    120
+}
+
+
+
 pub(crate) fn responses_sse_event(event_type: &str, data: serde_json::Value) -> Event {
     Event::default()
         .event(event_type)
-        .data(serde_json::to_string(&data).unwrap())
+        .data(serde_json::to_string(&data).unwrap_or_default())
 }
 
 // ─── Image extraction ────────────────────────────────────────────────────────