Store Token Activity in a bounded SQLite cache - #39

What changed

• keep Usage remaining as the default view and skip local rollout scans there
• import Token Activity into a local SQLite store only when the user opens it
• query bounded time ranges and filters instead of keeping full history in memory
• stream large rollout records and deduplicate resumed and forked task counters
• keep the existing usage-history sync unchanged

Measured on real data

• about 20 GB of rollout files
• cold import: about 6m39s
• import RSS: about 106–122 MB
• warm launch: about 2s and 111 MB RSS
• SQLite: 13 MB plus a 5 MB WAL
• 20,019-record restore test: about 419 ms and under 10 MB added RSS

Checks

• swift test: 549 tests, 1 skipped, 0 failures
• full-screen QA passed
• default Usage remaining view creates no local activity database

Scope

Usage per token, concurrency, receipts, and insights stay deferred. Cross-Mac Token Activity sync is tracked in #38 and blocks release acceptance for #36. Do not merge or release this draft until that gate passes.