Add reconnectable async chat-turn flow with in-memory TurnRegistry

Replace the one-shot SSE chat stream with an async dispatch + reconnectable
replay flow so the mobile client survives backgrounding, network blips, and
OS-killed sockets without losing an in-flight agentic turn.

- TurnRegistry/TurnEntry: in-memory per-turn event buffer (cap 500, front
  eviction) shared by the agentic loop (writer) and SSE replay readers.
  ReplayOutcome + replay_from/next_batch distinguish Events/CaughtUp/Gone;
  next_batch registers the Notify before reading state (no lost wakeup) and
  drains every buffered event before signaling terminal, so the final
  Done/Error is never dropped and the stream closes cleanly.
- Endpoints: POST /insights/chat/turn (202 + turn_id), GET
  /insights/chat/turn/{id} (SSE replay, ?skip_before= resume, per-event seq,
  410 on eviction), DELETE /insights/chat/turn/{id} (real task abort +
  cooperative is_running() check at each loop boundary).
- Cancellation actually stops the task (AbortHandle stored on the entry) and
  emits a Done{cancelled:true}; callers skip persistence on cancel.
- Background sweeper drops stale turns; interval clamped to <=300s.
- OpenTelemetry spans: ai.chat.turn.execute/replay/cancel.
- Legacy POST /insights/chat/stream path preserved unchanged.

Tests: registry coverage for terminal delivery (race guard), waiting, Gone,
abort, eviction; handler integration tests for 404/410, skip_before, seq
stamping, completed replay, and cancel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -66,6 +66,7 @@ image_hasher = "3.0"
 bk-tree = "0.5"
 async-trait = "0.1"
 indicatif = "0.17"
+uuid = { version = "1.10", features = ["v4", "serde"] }
 
 # Windows lacks system sqlite3, so re-enable the bundled C build there.
 # Linux/macOS use the system library (faster builds, smaller binary).