Create scheduled_messages for databases that never had it

No committed migration creates the table: migrations 6-10 were generated
outside the repository, so fresh databases fail once anything alters it.
The new migration is CREATE TABLE IF NOT EXISTS, a no-op where the table
already exists. Aerich cannot generate it because its saved state already
includes the table; the DDL is Tortoise's schema output for the model.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/models/6_20260603000000_add_scheduled_messages.py

@@ -0,0 +1,38 @@
+"""Create scheduled_messages, which no committed migration ever created.
+
+Migrations 6–10 were generated outside the repository and never committed, and
+one of them created this table. Deployments that ran them already have it, so
+this is IF NOT EXISTS and a no-op there; fresh databases need it before
+migration 20 alters the table. Aerich cannot generate this file because its
+saved model state already includes the table: the DDL below is Tortoise's
+schema output for the model as of migration 19.
+"""
+
+from tortoise import BaseDBAsyncClient
+
+RUN_IN_TRANSACTION = True
+
+
+async def upgrade(db: BaseDBAsyncClient) -> str:
+    return """
+        CREATE TABLE IF NOT EXISTS "scheduled_messages" (
+    "id" UUID NOT NULL PRIMARY KEY,
+    "recipient" VARCHAR(255) NOT NULL,
+    "channel" VARCHAR(20) NOT NULL,
+    "content" TEXT NOT NULL,
+    "subject" VARCHAR(255),
+    "scheduled_at" TIMESTAMPTZ NOT NULL,
+    "status" VARCHAR(20) NOT NULL DEFAULT 'pending',
+    "recurrence" VARCHAR(20) NOT NULL DEFAULT 'none',
+    "use_agent" BOOL NOT NULL DEFAULT False,
+    "error_message" TEXT,
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "created_by_id" UUID NOT NULL REFERENCES "users" ("id") ON DELETE CASCADE
+);"""
+
+
+async def downgrade(db: BaseDBAsyncClient) -> str:
+    # Existing deployments created this table in an uncommitted migration;
+    # never drop it from under them.
+    return ""