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

Critical bug fix: The loop variable 'block' was being reused for each
iteration, causing all elements in the returned slice to point to the
same memory location. This resulted in the last row being repeated for
every element when callers read the list.

Fixed by allocating a new block pointer for each iteration:
- Before: var block communities.CommunityBlock (reused)
- After: block := &communities.CommunityBlock{} (new allocation)

Also replaced fmt.Printf with log.Printf for consistency with project
logging standards.

Addresses: P1 review comment - pointer reuse in list operation

Change subscription lexicon subject field format from "at-identifier"
to "did" for consistency and correctness:

Before:
- format: "at-identifier" (accepts DIDs or handles)
- description: "DID or handle of the community"

After:
- format: "did" (only accepts DIDs)
- description: "DID of the community being subscribed to"

Rationale:
1. Matches block.json pattern (which correctly uses "did" format)
2. Aligns with service layer implementation (only supports DIDs)
3. Follows atProto convention: "subject" field references entities by DID
4. Prevents invalid handle values in federated records

This ensures subscription records are properly validated and compatible
with the broader atProto ecosystem.

E2E Tests (3 new test cases):
- Block via XRPC endpoint: Full flow from HTTP → PDS → Jetstream → AppView
- Unblock via XRPC endpoint: Complete unblock flow with DELETE event
- Block fails without authentication: Validates auth requirement (401)

Each E2E test verifies:
✓ XRPC endpoint responds correctly
✓ Record created/deleted on PDS
✓ Jetstream consumer indexes event
✓ AppView database state updated

Unit Test Updates:
- Added 6 mock methods to mockCommunityRepo for blocking operations
- Ensures service layer tests compile and pass

All tests follow existing E2E patterns (subscribe/unsubscribe) for
consistency.

Add 16 integration test cases covering:

1. Jetstream Consumer Indexing (4 tests):
- Block CREATE event indexing
- Block DELETE event indexing
- Idempotent duplicate event handling
- Graceful handling of non-existent block deletion

2. List Operations (3 tests):
- List all blocked communities for user
- Pagination with limit/offset
- Empty list for users with no blocks

3. IsBlocked Queries (3 tests):
- Returns false when not blocked
- Returns true when blocked
- Returns false after unblock

4. GetBlock Operations (3 tests):
- Error when block doesn't exist
- Retrieve block by user DID + community DID
- Retrieve block by AT-URI (for DELETE operations)

All tests verify proper database state, idempotency guarantees,
and Jetstream event processing.