refactor(backend): change prompt field type from varchar(8192) to text

Update the 'prompt' field in the Assistant model to use a Text type for improved flexibility. This change is accompanied by a migration script to alter the database schema accordingly.

backend/migrations/versions/20260807_0014_assistant_prompt_to_text.py

@@ -0,0 +1,40 @@
+"""change assistants.prompt from varchar(8192) to text
+
+Revision ID: 20260807_0014
+Revises: 20260807_0013
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "20260807_0014"
+down_revision: str | Sequence[str] | None = "20260807_0013"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.alter_column(
+        "assistants",
+        "prompt",
+        existing_type=sa.String(length=8192),
+        type_=sa.Text(),
+        existing_nullable=False,
+        existing_server_default=sa.text("''"),
+    )
+
+
+def downgrade() -> None:
+    op.alter_column(
+        "assistants",
+        "prompt",
+        existing_type=sa.Text(),
+        type_=sa.String(length=8192),
+        existing_nullable=False,
+        existing_server_default=sa.text("''"),
+    )