feat(go-services): Phase 2 — combat_stats accumulator (cross-language exact)

Ports main.py's _combat_session_delta / _combat_merge_into_lifetime (incl. the
documented "offense/defense use latest, additively" quirk) and the combat_stats
handler (session delta -> DB-backed lifetime merge -> delete-then-insert of
combat_stats + combat_stats_sessions). Read handlers gain the live combat
overlay (union live + DB), like Python.

Validation:
- combat.go `combat-merge` CLI folds snapshots through the accumulator; diffed
  against the Python functions on identical input -> byte-IDENTICAL.
- combat_test.go golden test runs in the build (go test now part of the tracker
  Dockerfile).
- Live: 40 combat lifetime rows + 40 session snapshots + rare_events flowing in
  dereth_go via the shadow consumer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go-services/tracker-go/Dockerfile

@@ -8,6 +8,7 @@ WORKDIR /src
 # reads the imports from the source and writes go.mod/go.sum, then we build.
 COPY . .
 RUN go mod tidy
+RUN go test ./...
 ARG BUILD_VERSION=dev
 RUN CGO_ENABLED=0 GOOS=linux go build \
     -trimpath \