[P1] Scale shard processing and bound hot-path concurrency

[apple050620312]

Summary

• Add validated SHARD_COUNT and SHARD_IDS configuration for distributed shard workers.
• Move synchronous database operations off the event loop with bounded execution time.
• Add bounded TTL/LRU caches for guild settings, filters, and webhooks.
• Bound link-fix and EmbedEZ concurrency and rate-limit overload warnings.
• Replace analytics row loading with database-side counts and buffered asynchronous event writes.
• Document multi-process shard assignment and capacity-tuning controls.

Priority

P1 — removes event-loop and upstream-service bottlenecks that become critical at roughly 50,000 guilds.

Dependency

Stacked on #1. Review and merge #1 first; after it merges, retarget this PR to main.

Validation

• Shard parsing and cache behavior tests pass.
• Runtime import and source-tree parsing checks pass.

[apple050620312]

Superseded by upstream PR Kyrela#112.