Canonical v2-schema `.golangci.yml`, golangci-lint pins bumped to v2.12.2 in `Dockerfile` and `script/bootstrap`, and the tree brought to `0 issues.` under it.
Three behaviour deltas: `Cache.StoreVariant` takes a context (cancelled requests skip the accounting row, recovered by reconciliation); `MetadataStorage.Store` no longer leaks `.tmp-*.json` on Write/Close/Rename failure (dead-defer bug fix); the `signing_key` too-short error text gained a `value too short:` prefix.
Eviction-loop context cancellation deferred to #102.
- Make ExpiresAt optional in CBOR (omitempty) for smaller tokens
- Treat ExpiresAt=0 as 'never expires' in parser
- URL-encode token with url.PathEscape() for safety
- Add 'Never' as default TTL option in generator form
