feat(api): add LLM model configurations endpoint and schemas

- Introduce new API route `/access` for managing LLM model configurations
- Add LLMModelConfig model to database models
- Include comprehensive schemas for LLM model config CRUD operations
- Register new router with tags for LLM Model Configurations

gns3server/db_migrations/versions/20260303_create_llm_model_configs_table.py

@@ -0,0 +1,62 @@
+"""create llm_model_configs table
+
+Revision ID: 20260303_create_llm_model_configs
+Revises: 7ceeddd9c9a8
+Create Date: 2026-03-03
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision = '20260303_create_llm_model_configs'
+down_revision = '7ceeddd9c9a8'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Create llm_model_configs table
+    op.create_table(
+        'llm_model_configs',
+        sa.Column('config_id', postgresql.UUID(as_uuid=True), primary_key=True),
+        sa.Column('config', postgresql.JSONB(), nullable=False),
+        sa.Column('user_id', postgresql.UUID(as_uuid=True), sa.ForeignKey('users.user_id', ondelete='CASCADE'), nullable=True),
+        sa.Column('group_id', postgresql.UUID(as_uuid=True), sa.ForeignKey('user_groups.user_group_id', ondelete='CASCADE'), nullable=True),
+        sa.Column('is_default', sa.Boolean(), default=False, nullable=False),
+        sa.Column('created_at', sa.DateTime(), nullable=True),
+        sa.Column('updated_at', sa.DateTime(), nullable=True),
+        sa.CheckConstraint(
+            "(user_id IS NOT NULL AND group_id IS NULL) OR "
+            "(user_id IS NULL AND group_id IS NOT NULL)",
+            name='single_owner_check'
+        ),
+        sa.UniqueConstraint(
+            'user_id', 'is_default',
+            name='unique_user_default',
+            deferrable=True, initially='deferred',
+            postgresql_where=sa.text("is_default = TRUE AND user_id IS NOT NULL")
+        ),
+        sa.UniqueConstraint(
+            'group_id', 'is_default',
+            name='unique_group_default',
+            deferrable=True, initially='deferred',
+            postgresql_where=sa.text("is_default = TRUE AND group_id IS NOT NULL")
+        ),
+    )
+
+    # Create indexes for efficient queries
+    op.create_index('idx_llm_model_configs_user_id', 'llm_model_configs', ['user_id'])
+    op.create_index('idx_llm_model_configs_group_id', 'llm_model_configs', ['group_id'])
+    op.create_index('idx_llm_model_configs_config', 'llm_model_configs', ['config'], postgresql_using='gin')
+
+
+def downgrade() -> None:
+    # Drop indexes
+    op.drop_index('idx_llm_model_configs_config', table_name='llm_model_configs')
+    op.drop_index('idx_llm_model_configs_group_id', table_name='llm_model_configs')
+    op.drop_index('idx_llm_model_configs_user_id', table_name='llm_model_configs')
+
+    # Drop table
+    op.drop_table('llm_model_configs')