review: a cancelled turn keeps its trace, and a quiet box still expires (V-629)

Two defects found reviewing the PR.

The insert ran on the turn's own context, so a caller that hung up or timed out
cancelled it. That is exactly the turn worth having. It now runs detached, with
a one-second bound of its own, because a write must not hold the reply.

Retention was enforced on write alone, so a box that goes quiet for a month kept
every row until the next sixty-fourth turn. pruneTracesOnStart closes that, and
RoutingTraceRetention is exported so the daemon reads the same number the store
enforces.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117tgnmbgZpHVV3XSNw8Qua

docs/plans/21-persisting-the-routing-trace.md

@@ -2,9 +2,9 @@
 
 **Owner's call, 06-08-2026. Vikunja #629, umbrella #628.**
 
-**Verdict: the per-turn decision record is now written to the database.** That reverses a
-written decision, which is the point of this file. It is not an incidental telemetry
-feature and must not be read as one.
+**Verdict: the per-turn decision record now persists.** That reverses a written decision,
+which is the point of this file. It is not an incidental telemetry
+feature. Do not read it as one.
 
 Last verified: 06-08-2026 @ 799cf55
 
@@ -27,7 +27,7 @@ So the choice was between no routing heads and a persisted trace. The owner chos
 
 ## Retention, and why it is two answers
 
-**Raw trace: 14 days.** `routingTraceRetention` in `internal/store/routingtraces.go`. That
+**Raw trace: 14 days.** `store.RoutingTraceRetention` in `internal/store/routingtraces.go`. That
 is the life of a diagnosis with room for a weekend. The bound is an age and not a row
 count. The useful question is what she did this week, and a busy Tuesday must not push last
 Friday out.
@@ -45,8 +45,10 @@ support, and making it would be worse than staying silent.
 
 - **Nothing here leaves the box.** The rule that the owner's notes and facts are never
   search input covers this table too. No query source reads it, and no upstream engine can.
-- **Retention is enforced on write.** `WriteRoutingTrace` prunes every 64th row, which is
-  hours at human rate.
+- **Retention is enforced on write and again at start.** `WriteRoutingTrace` prunes every
+  64th row, which is hours at human rate. `pruneTracesOnStart` covers the case write alone
+  cannot. A box that goes quiet keeps every row until the next sixty-fourth turn. Without
+  the start-time prune, the bound would hold only for a box in daily use.
 - **Deletion already exists.** `Store.Wipe` drops every table the database reports, so
   `mavend -wipe -confirm-wipe` covers this one with no list to edit.
 - **The ring did not move.** It is still what `/trace` reads and still what a test with no