Test isolation bug fixed; stats now shows real ingestion health

Patch Changes

• 7fe55de: source backfill --dry-run now reports how much of the extracted history the source doesn't already have, as N not yet stored. Previously the dry-run line reported only how many entries were on the page, which said nothing about whether any of them were missing — during the 2026-07-23 ingest outage that gap let a run over a source missing its entire recent history read as uneventful. Requires an API worker carrying the new notStored field; against an older server the clause is omitted rather than guessed. The report type also catches up with the server: inserted is null on a dry run (nothing was written, no count computed) rather than a fabricated 0.
• b124608: Fix getApiUrl() memoizing the API base URL process-wide on first call. Under bun test, every test file shares one process, so whichever file called it first locked the base URL for the rest of the run — files that set RELEASES_API_URL afterward got the stale (production) URL instead and their assertions failed. This was silently CI-red on main (22 tests failing) because CI's clean environment produces a call order that trips the memoization, while a developer machine with real credentials hits a different, unrelated set of failures. getApiUrl() now re-resolves from the environment on every call instead of caching.
• f7a1b54: Fix releases stats reporting every source as never-fetched with zero recent releases. The command composed its summary from the legacy flat fields of /v1/stats and hardcoded source health, releasesInPeriod, and every lastFetchedAt to 0/null — a stale workaround from before the endpoint returned the full StatsSummary shape. It now reads the server's real values, so the output reflects actual ingestion health instead of implying the index is dead.