feat(store): fetch contributions incrementally and persist history in SQLite

Bound each provider's startFetch()/resolveFetch() to an optional
since/until window, wire a SQLite-backed ContributionStore into
ContributionAggregator (fetch only the trailing window past the last
stored date, merge into the store, prune by CONTRIBUTIONS_RETENTION_DAYS),
and add a graph:contributions:refetch command to force a full or ranged
re-fetch in <=365-day chunks. This is the root fix for the full
365-day-refetch timeout that used to hit on every cache miss.

.env

@@ -19,3 +19,7 @@ GITLAB_URL=
 GITEA_USER=
 GITEA_TOKEN=
 GITEA_URL=
+
+# Number of days of contribution history to keep in var/data/contributions.db.
+# 0 (or unset) keeps history forever.
+CONTRIBUTIONS_RETENTION_DAYS=0