Improve mutation channel robustness

- Add event type constants (MutationCreate, MutationUpdate, MutationDelete, MutationComment)
- Make buffer size configurable via BEADS_MUTATION_BUFFER (default 512, up from 100)
- Add defensive closed-channel handling in event loop consumer
- Add 1s dropped-events ticker (down from 60s) for faster recovery
- Verify mutationChan is never closed (prevents panic)

Oracle review findings addressed:
- Eliminates panic risk from send-on-closed-channel
- Reduces worst-case recovery latency from 60s to 1s
- Increases buffer capacity for better burst handling
- Type-safe event constants prevent string typos

Related: bd-36320a04, bd-1f4086c5

internal/rpc/server_issues_epics.go

@@ -161,7 +161,7 @@ func (s *Server) handleCreate(req *Request) Response {
 	}
 
 	// Emit mutation event for event-driven daemon
-	s.emitMutation("create", issue.ID)
+	s.emitMutation(MutationCreate, issue.ID)
 
 	data, _ := json.Marshal(issue)
 	return Response{
@@ -195,7 +195,7 @@ func (s *Server) handleUpdate(req *Request) Response {
 	}
 
 	// Emit mutation event for event-driven daemon
-	s.emitMutation("update", updateArgs.ID)
+	s.emitMutation(MutationUpdate, updateArgs.ID)
 
 	issue, err := store.GetIssue(ctx, updateArgs.ID)
 	if err != nil {
@@ -232,7 +232,7 @@ func (s *Server) handleClose(req *Request) Response {
 	}
 
 	// Emit mutation event for event-driven daemon
-	s.emitMutation("update", closeArgs.ID)
+	s.emitMutation(MutationUpdate, closeArgs.ID)
 
 	issue, _ := store.GetIssue(ctx, closeArgs.ID)
 	data, _ := json.Marshal(issue)