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/migrations/012_moderation.sql

@@ -0,0 +1,20 @@
+-- Moderation tables: reports and bans.
+
+CREATE TABLE IF NOT EXISTS reports (
+    id                  INTEGER PRIMARY KEY AUTOINCREMENT,
+    encrypted_report    BLOB NOT NULL,
+    conversation_id     BLOB NOT NULL,
+    reporter_identity   BLOB NOT NULL,
+    created_at          INTEGER NOT NULL DEFAULT (strftime('%s', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_reports_created ON reports(created_at);
+
+CREATE TABLE IF NOT EXISTS bans (
+    identity_key    BLOB PRIMARY KEY,
+    reason          TEXT NOT NULL DEFAULT '',
+    banned_at       INTEGER NOT NULL DEFAULT (strftime('%s', 'now')),
+    expires_at      INTEGER NOT NULL DEFAULT 0
+);
+
+CREATE INDEX IF NOT EXISTS idx_bans_expires ON bans(expires_at);