test: add comprehensive thumb validation and blob transformation tests

Add extensive test coverage for external embed thumb validation and blob
reference transformation in feed responses.

**Thumb Validation Tests** (post_thumb_validation_test.go):
7 test cases covering strict blob reference validation:
1. ❌ Reject thumb as URL string (must be blob ref)
2. ❌ Reject thumb missing $type field
3. ❌ Reject thumb missing ref field
4. ❌ Reject thumb missing mimeType field
5. ✅ Accept valid blob reference
6. ✅ Accept missing thumb (unfurl will handle)
7. Security: Prevents URL injection attacks via thumb field

**Feed Blob Transform Tests** (feed_test.go):
6 test cases for GetCommunityFeed blob URL transformation:
1. Transforms blob refs to PDS URLs
2. Preserves community PDSURL in PostView
3. Generates correct getBlob endpoint URLs
4. Handles posts without embeds
5. Handles posts without thumbs
6. End-to-end feed query validation

**Integration Test Updates:**
- Update post creation tests for content length validation
- Update post handler tests with proper context setup
- Update E2E tests for nested external embed structure
- Add helper for creating communities with PDS credentials
- Add createTestUser helper for unique test isolation

**Test Isolation:**
- Use unique DIDs per test (via t.Name() suffix)
- Prevent cross-test data contamination
- Proper cleanup with defer db.Close()

**Example Validation:**
```go
// ❌ This should fail validation:
"thumb": "https://example.com/thumb.jpg" // URL string

// ✅ This should pass validation:
"thumb": {
"$type": "blob",
"ref": {"$link": "bafyrei..."},
"mimeType": "image/jpeg",
"size": 52813
}
```

**Coverage:**
- Blob validation: 7 test cases
- Blob transformation: 6 test cases
- Feed integration: 99 added lines in feed_test.go
- Total: 13 new test scenarios

This ensures security (no URL injection), correctness (proper blob format),
and functionality (URLs work in API responses).