fix: Hot-reload pricing table when a newer snapshot lands

PricingService loaded the litellm pricing table from disk once per app
launch (a loaded-guard) and never reloaded. The background refresh
downloads a fresh snapshot to disk on a 24h TTL, but nothing pulled it
back into memory, so a long-running session (the menu bar app can run
for weeks) stayed pinned to the snapshot present at launch. A model
released mid-session (e.g. opus-4-8) had no pricing entry and every one
of its rows priced at $0 — today's cost collapsed to the non-4-8
remainder (observed $0.25 vs a correct ~$13).

Add reloadIfFreshChanged(): each refresh cycle, if the on-disk fresh
file's mtime differs from the in-memory snapshot's, reload it. Wired
into SessionParseCacheV2.refreshFromFilesystem() before pricing rows,
so a fresh download takes effect within one cycle (~5 min), no restart.

Also resolve all model prices in a single atomic actor hop up front in
costSummary() (new prices(for:)) instead of awaiting per row, so a
concurrent reload can't swap the table mid-summary and mix old/new
prices into one result.

Verified end-to-end: with opus-4-8 removed from the on-disk table the
running app showed $0.49; swapping the full table back in, it
hot-reloaded to the correct total within one cycle without a restart.