Self-serve webhooks; personal feed now cursor-paginated

Added

• Self-serve webhook management — create, list, update, and delete subscriptions from Account → Notifications with org-scoped or follows-scoped targeting, per-event product and release-type filters, delivery activity history, and auto-pause on repeated failures with an owner email alert when a subscription is paused.

Changed

• Feed rows with multiple image attachments now display a compact bottom gallery of chips; single-attachment rows keep the existing side-rail preview.
• Following feed now renders immediately on page load — the first page is server-prefetched and cached (~90 s TTL) so the timeline appears without a client-side loading flash.
• Personalized feed API (GET /v1/me/feed) switched to cursor pagination; the ?page= parameter is rejected with 400.

Fixed

• Product and source bylines no longer display version labels for codename-versioned releases — only numeric semver/calver strings appear; model codenames (e.g. "Opus 4.7") are omitted.