chore: remove stale TODO, update codecov-action to v5, refresh tech debt doc

The guild daily RP rollover TODO in handlers_guild_ops.go was stale —
the feature was already implemented via lazy rollover in
handlers_guild.go. Several other items in technical-debt.md were also
resolved in prior commits (typos, db guard investigation). Updated
the doc to reflect current state and bumped codecov-action to v5.

.github/workflows/go.yml

@@ -44,7 +44,7 @@ jobs:
       run: go test -race -coverprofile=coverage.out ./... -timeout=10m
 
     - name: Upload Coverage to Codecov
-      uses: codecov/codecov-action@v4
+      uses: codecov/codecov-action@v5
       with:
         files: ./coverage.out
         flags: unittests