[scanner] Lemmy / fediverse scanner

[sausi-7]

Summary

Many niche communities have migrated to Lemmy instances. Each instance exposes a JSON API; the scanner should accept an instance URL + a list of communities to watch.

Same protocol as the Reddit scanner — just a different endpoint. Scope is larger than the HN/PH issues because auth / instance discovery adds a little friction; hence help wanted in addition to good first issue.

Files to touch

• new: inkwell/scanners/lemmy.py
• edit: inkwell/scanners/registry.py
• new: tests/test_scanners/test_lemmy.py

Open questions

• How to handle federated content (the same post visible on multiple instances)? Dedup by original ap_id.
• Should we default to a specific instance (lemmy.world)? Or require the user to pass one?

See CONTRIBUTING.md → How to add a new scanner.