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/error_codes.rs

@@ -33,6 +33,12 @@ pub const E027_BLOB_NOT_FOUND: &str = "E027";
 pub const E028_ACCOUNT_DELETION_FAILED: &str = "E028";
 pub const E029_DEVICE_LIMIT: &str = "E029";
 pub const E030_DEVICE_NOT_FOUND: &str = "E030";
+#[allow(dead_code)] // used by v2 RPC moderation handlers
+pub const E031_USER_BANNED: &str = "E031";
+#[allow(dead_code)] // used by v2 RPC moderation handlers
+pub const E032_REPORT_EMPTY: &str = "E032";
+#[allow(dead_code)] // used by v2 RPC moderation handlers
+pub const E033_ADMIN_REQUIRED: &str = "E033";
 
 /// Build a `capnp::Error::failed()` with the structured code prefix.
 pub fn coded_error(code: &str, msg: impl std::fmt::Display) -> capnp::Error {