Move ai_guard_violations migration to migrations/ dir picked up by db-migrate

migrations_new/ is not read by db-migrate (it only honors MIGRATIONS_DIR,
which defaults to /migrations, i.e. crates/db/migrations). The file was
misplaced and would never have run.

crates/db/migrations/20260815000001_create_ai_guard_violations.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS ai_guard_violations;

crates/db/migrations/20260815000001_create_ai_guard_violations.up.sql

@@ -0,0 +1,16 @@
+-- AI Guard Violations — records every message blocked by the content guard.
+-- Migration: 20260815000001
+
+CREATE TABLE IF NOT EXISTS ai_guard_violations (
+    id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id      UUID REFERENCES users(id) ON DELETE SET NULL,
+    message_excerpt TEXT NOT NULL,          -- first 200 chars, never the full text
+    guard_type   VARCHAR(40) NOT NULL,      -- 'keyword' | 'length' | 'flood' | 'moderation_api'
+    reason       TEXT NOT NULL,             -- human-readable rejection reason
+    categories   JSONB,                     -- OpenAI moderation categories (moderation_api only)
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_ai_guard_violations_user_id    ON ai_guard_violations(user_id);
+CREATE INDEX IF NOT EXISTS idx_ai_guard_violations_created_at ON ai_guard_violations(created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_ai_guard_violations_guard_type ON ai_guard_violations(guard_type);