- Fix P1 issue: properly bubble up database errors instead of masking as conflict
* Only return ErrBlockAlreadyExists when getErr is ErrBlockNotFound (race condition)
* Real DB errors (outages, connection failures) now propagate to operators
- Remove unused V1 functions flagged by linter:
* createRecordOnPDS, deleteRecordOnPDS, callPDS (replaced by *As versions)
- Apply automatic code formatting via golangci-lint --fix:
* Align struct field tags in CommunityBlock
* Fix comment alignment across test files
* Remove trailing whitespace
- All tests passing, linter clean

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Fixes four issues identified in PR review:

**BUG 1 - Performance: Remove redundant database query**
- Removed duplicate GetByDID call in BlockCommunity service method
- ResolveCommunityIdentifier already verifies community exists
- Reduces block operations from 2 DB queries to 1

**BUG 2 - Performance: Move regex compilation to package level**
- Moved DID validation regex to package-level variable in block.go
- Prevents recompiling regex on every block/unblock request
- Eliminates unnecessary CPU overhead on hot path

**BUG 3 - DRY: Remove duplicated extractRKeyFromURI**
- Removed duplicate implementations in service.go and tests
- Now uses shared utils.ExtractRKeyFromURI function
- Single source of truth for AT-URI parsing logic

**P1 - Critical: Fix duplicate block race condition**
- Added ErrBlockAlreadyExists error type
- Returns 409 Conflict instead of 500 when PDS has block but AppView hasn't indexed yet
- Handles normal race in eventually-consistent flow gracefully
- Prevents double-click scenarios from appearing as server failures

All tests passing (33.2s runtime, 100% pass rate).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

**Breaking Change**: XRPC endpoints now strictly enforce lexicon spec.

Changed endpoints to reject handles and accept ONLY DIDs:
- social.coves.community.blockCommunity
- social.coves.community.unblockCommunity
- social.coves.community.subscribe
- social.coves.community.unsubscribe

Rationale:
1. Lexicon defines "subject" field with format: "did" (not "at-identifier")
2. Records are immutable and content-addressed - must use permanent DIDs
3. Handles can change (they're DNS pointers), DIDs cannot
4. Bluesky's app.bsky.graph.block uses same pattern (DID-only)

Previous behavior accepted both DIDs and handles, resolving handles to
DIDs internally. This was convenient but violated the lexicon contract.

Impact:
- Clients must resolve handles to DIDs before calling these endpoints
- Matches standard atProto patterns for block/subscription records
- Ensures federation compatibility

This aligns our implementation with the lexicon specification and
atProto best practices.

Improve validation robustness in block/unblock handlers:

1. DID validation with regex:
- Pattern: ^did:(plc|web):[a-zA-Z0-9._:%-]+$
- Rejects invalid formats like "did:x" or "did:"
- Ensures only supported DID methods (plc, web)

2. Handle validation:
- Verify handle contains @ symbol for domain
- Rejects incomplete handles like "!" or "!name"
- Ensures proper format: !name@domain.tld

Previous validation only checked prefix, allowing invalid values
to pass through to service layer. New validation catches format
errors early with clear error messages.

Addresses: Important review comment #4

Service layer improvements:

1. Add DID verification in ResolveCommunityIdentifier:
- When a DID is provided, verify the community actually exists
in the AppView database
- Prevents accepting non-existent DIDs (e.g., did:plc:fakefake)
- Provides clearer error messages when community doesn't exist

2. Improve duplicate error detection in BlockCommunity:
- Check for HTTP 409 Conflict status code explicitly
- Added "status 409" check in addition to text-based detection
- More robust across different PDS implementations
- Still maintains fallback checks for compatibility

Both changes improve error handling and user experience while
maintaining backward compatibility.

Addresses: Critical review comment #2, Important review comment #3

Database optimization changes:

1. Removed redundant idx_blocks_user_community index:
- UNIQUE constraint on (user_did, community_did) already creates
an index automatically
- Maintaining duplicate index wastes storage and degrades write
performance (every insert updates two identical indexes)

2. Added missing idx_blocks_record_uri index:
- Required for GetBlockByURI() queries used in Jetstream DELETE
operations
- Without this index, DELETE event processing does full table scan

Migration now has optimal indexes without redundancy.

Addresses: Critical review comments #1 and #7