feat: add abuse prevention and moderation (Phase 5.6)

Add server-side moderation service with report submission, user
banning/unbanning, and admin listing endpoints. Add client-side
user blocking with message filtering in ConversationStore.

Server:
- ModerationService domain logic (report, ban, unban, list)
- Storage trait methods + FileBackedStore + SqlStore implementations
- SQL migration 012_moderation.sql (reports + bans tables)
- Error codes E031-E033 for moderation
- Domain types for all moderation request/response pairs
- 10 new tests (6 domain + 4 storage)

SDK:
- blocked_users table in ConversationStore
- block_user, unblock_user, is_blocked, list_blocked methods
- load_recent_messages_filtered excludes blocked senders
- QpqClient moderation convenience methods
- 4 new tests for block/unblock/filter

crates/quicproquo-server/src/domain/mod.rs

@@ -13,5 +13,8 @@ pub mod channels;
 pub mod users;
 pub mod blobs;
 pub mod devices;
+pub mod groups;
 pub mod p2p;
 pub mod account;
+pub mod moderation;
+pub mod recovery;