feat(fuse): add metadata cache to reduce filesystem lookups

[javi11]

Summary

• Add application-level metadata caching for FUSE mount to reduce filesystem/metadata service queries
• Implement LRU caches with TTL for stat results, directory listings, and negative lookups
• Add cache invalidation integration with import service via FUSECacheNotifier interface
• Add frontend UI for configuring metadata cache settings

Changes

• New internal/fuse/cache/ package: Cache interface, LRU implementation, and tests
• Config updates: Add metadata cache configuration options with sensible defaults
• FUSE adapter integration: Cache checks in Getattr, Lookup, Readdir; invalidation on Rename
• Import service integration: Cache invalidation after successful imports
• Frontend UI: Metadata Cache Settings section in FUSE configuration page

Test plan

• Unit tests for cache package (go test ./internal/fuse/cache/...)
• Existing FUSE adapter tests pass (go test ./internal/fuse/...)
• Frontend TypeScript compilation passes (bun run check)
• Manual testing: Enable cache, verify UI controls work, test cache behavior

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