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.

config/services.yaml

@@ -9,6 +9,7 @@ parameters:
     env(GITEA_USER): ""
     env(GITEA_TOKEN): ""
     env(GITEA_URL): ""
+    env(CONTRIBUTIONS_RETENTION_DAYS): "0"
 
 services:
     _defaults: