perf: Cache project lookup

[Luca Forstner (lforst)]

• Reduce repeated project lookup requests by sharing a bounded cache across loggers and imported spans. Previously, each could independently resolve the same project; now concurrent requests share one lookup, and cached results expire after 15 minutes.
• Deduplicate simultaneous login requests during startup. Previously, callers could each send a login request before the first completed; now matching callers wait for the same request.
• Preserve explicit forced logins, retry failed requests, and clear project caches when login context changes. Existing loggers and spans keep their resolved project IDs.