Wire unfurl and blob services into the post creation pipeline, enabling automatic enhancement of external embeds with rich metadata and thumbnails. **Post Service Integration:** - Add optional BlobService and UnfurlService dependencies - Update constructor to accept blob/unfurl services (nil-safe) - Add ThumbnailURL field to CreatePostRequest for client-provided URLs - Add PDSURL to CommunityRef for blob URL transformation (internal only) **Server Main Changes:** - Initialize unfurl repository with PostgreSQL - Initialize blob service with default PDS URL - Initialize unfurl service with: - 10s timeout for HTTP fetches - 24h cache TTL - CovesBot/1.0 user agent - Pass blob and unfurl services to post service constructor **Flow:** ``` Client POST → CreateHandler ↓ PostService.Create() [external embed detected] ↓ (if no thumb provided) UnfurlService.UnfurlURL() [fetch oEmbed/OpenGraph] ↓ (cache miss) HTTP fetch → oEmbed provider / HTML parser ↓ (thumbnail URL found) BlobService.UploadBlobFromURL() [download & upload to PDS] ↓ com.atproto.repo.uploadBlob → PDS ↓ (returns BlobRef with CID) Embed enriched with thumb blob → Write to PDS ``` **Interface Documentation:** - Added comments explaining optional blob/unfurl service injection - Unfurl service auto-enriches external embeds when provided - Blob service uploads thumbnails from unfurled URLs This is the core integration that enables the full unfurling feature. The actual unfurl logic in posts/service.go will be implemented separately.