feat(db): F3a prompt_templates 表 + 迁移

新增 PromptTemplate 模型（UuidPk+CreatedAt+Base；owner_id stub、title、body、
category、tool_key）+ alembic 迁移。alembic check 无漂移；ruff/mypy packages/db 干净。

packages/db/migrations/versions/59a9198d4604_prompt_templates_table.py

@@ -0,0 +1,53 @@
+"""prompt_templates table
+
+Revision ID: 59a9198d4604
+Revises: d3e4f5a6b7c8
+Create Date: 2026-06-23 20:10:37.869872
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "59a9198d4604"
+down_revision: str | None = "d3e4f5a6b7c8"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table(
+        "prompt_templates",
+        sa.Column("owner_id", sa.Uuid(), nullable=False),
+        sa.Column("title", sa.Text(), nullable=False),
+        sa.Column("body", sa.Text(), nullable=False),
+        sa.Column("category", sa.Text(), nullable=True),
+        sa.Column("tool_key", sa.Text(), nullable=True),
+        sa.Column("id", sa.Uuid(), server_default=sa.text("gen_random_uuid()"), nullable=False),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            nullable=False,
+        ),
+        sa.ForeignKeyConstraint(
+            ["owner_id"],
+            ["users.id"],
+        ),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index(
+        op.f("ix_prompt_templates_owner_id"), "prompt_templates", ["owner_id"], unique=False
+    )
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f("ix_prompt_templates_owner_id"), table_name="prompt_templates")
+    op.drop_table("prompt_templates")
+    # ### end Alembic commands ###