feat: integrate unfurl and blob services into post creation flow

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.