Merge braindump content into knowledge base

[gcomte]

Summary

• Add new documentation pages: AppSpecs, PubkyApp client, SecurityModel, HTTPRelay, PubkyBackup, PubkyModeration
• Standardize PKARR casing (Pkarr → PKARR) throughout all docs
• Reduce content duplication across SecurityModel, Authentication, Homeserver, and PubkyBackup
• Improve link text for GitHub references (e.g. AUTH.md → pubky-core GitHub docs)
• Clean up terminology and phrasing (credible exit, cold key → key isolation, relay security, etc.)
• Remove Philosophy.md (content to be revisited later)
• Trim DHT Attackers section in SecurityModel to link external references

Test plan

• Split up to more smaller PRs that can be reviewed by the right team.
• Verify all wikilinks resolve correctly in Quartz
• Check that no broken links were introduced
• Review new pages render properly

[gcomte]

Superseded by #45, #46, #47, #48, #49.