fix: improve chat storage scaling and privacy

[shakkernerd]

Summary

• add private durable event storage with events.is_private and event_recipients
• tighten DM access for replay, search, threads, reactions, edits, deletes, uploads, and read receipts
• add root-message-only paging/read semantics for DMs so thread replies do not affect timeline unread counts
• add event pruning with retention bounds and safe timestamp cutoff handling
• batch DM member and attachment hydration, plus add indexes for message paging, replay, and hydration hot paths
• update docs, OpenAPI, SDK search helper, and embedded web assets

Verification

• go test ./apps/api/internal/store/sqlite -run 'TestMessagePrivacyScaling' -count=1\n- go test ./apps/api/internal/store/sqlite -run 'TestMessagePrivacyScalingEventPruningUsesTimestampCutoff' -count=1 -v\n- pnpm check\n- pnpm coverage\n- git diff --check