feat: add ssh_key_id to config profiles for container key mounting

- Add ssh_key_id column to ConfigProfile model and migration
- Update config profile API to accept/return ssh_key_id
- Include ssh_key_id in ResolvedProfile and resolver logic
- Mount selected SSH key into container home dir at start_instance
- Frontend config profile form with SSH key selector dropdown
- Git mount URL validation defaults to profile's SSH key

Quality gates: pytest (231 passed, 6 pre-existing), tsc --noEmit clean

apps/api/alembic/versions/069d3da4dc9b_add_ssh_key_id_to_config_profiles.py

@@ -0,0 +1,32 @@
+"""add_ssh_key_id_to_config_profiles
+
+Revision ID: 069d3da4dc9b
+Revises: 2026_05_29_add_notifications_table
+Create Date: 2026-05-29 12:30:16.580532
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "069d3da4dc9b"
+down_revision = "2026_05_29_add_notifications_table"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "config_profiles",
+        sa.Column(
+            "ssh_key_id",
+            sa.Uuid(),
+            sa.ForeignKey("ssh_keys.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("config_profiles", "ssh_key_id")