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.

src/Service/Provider/ProviderInterface.php

@@ -6,17 +6,33 @@ namespace GitContributionGraph\Service\Provider;
 
 interface ProviderInterface
 {
-    /** Fire the HTTP request(s) without blocking; returns an opaque handle for resolveFetch(). */
-    public function startFetch(): mixed;
+    /**
+     * Fire the HTTP request(s) without blocking; the result is read later by
+     * resolveFetch() so multiple providers' requests can be in flight at once.
+     *
+     * @param  ?\DateTimeImmutable $since lower bound (inclusive); defaults to 365 days ago
+     * @param  ?\DateTimeImmutable $until upper bound (inclusive); defaults to now
+     * @return mixed opaque handle to pass into resolveFetch()
+     */
+    public function startFetch(?\DateTimeImmutable $since = null, ?\DateTimeImmutable $until = null): mixed;
 
-    /** @return array<string, int>  date (Y-m-d) => contribution count */
+    /**
+     * Block on the handle from startFetch() and parse it into a per-day count map.
+     *
+     * @param  mixed $handle the value returned by startFetch()
+     * @return array<string, int> date (Y-m-d) => contribution count
+     */
     public function resolveFetch(mixed $handle): array;
 
+    /** Whether the required credentials/URL for this provider are all set. */
     public function isConfigured(): bool;
 
+    /** Short lowercase identifier for this provider, e.g. "github". */
     public function getName(): string;
 
+    /** Check reachability/credentials without fetching contribution data; used by the /health endpoint. */
     public function probe(): ProviderStatus;
 
+    /** Make a lightweight authenticated request to verify the provider is reachable; throws on failure. */
     public function ping(): void;
 }