feat: implement tool instances backend and session navigation

Backend:
- Create ToolInstance model with status tracking
- Add Alembic migration for tool_instances table
- Create Docker service for compose template rendering and container execution
- Add CRUD API endpoints for tool instances
- Add lifecycle endpoints (start/stop/restart)
- Add user sessions endpoint for navigation
- Register routers in main.py

Frontend:
- Create SessionsProvider with React context
- Create sessions API client
- Update AppShell with sessions section in navigation
- Add session status indicators and polling
- Add CSS for session navigation

Quality gates: typecheck ✓, lint ✓, build ✓

apps/api/alembic/versions/0006_tool_instances.py

@@ -0,0 +1,55 @@
+"""add tool_instances table
+
+Revision ID: 0006_tool_instances
+Revises: 0005_ssh_keys_timestamps
+Create Date: 2026-05-19 10:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = "0006_tool_instances"
+down_revision: Union[str, None] = "0005_ssh_keys_timestamps"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "tool_instances",
+        sa.Column("id", postgresql.UUID(as_uuid=True), server_default=sa.text("gen_random_uuid()"), nullable=False),
+        sa.Column("name", sa.String(255), nullable=False),
+        sa.Column("display_name", sa.String(255), nullable=False),
+        sa.Column("tool_type_id", postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column("repository_id", postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column("project_id", postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column("owner_id", postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column("status", sa.String(50), nullable=False, server_default="pending"),
+        sa.Column("container_id", sa.String(255), nullable=True),
+        sa.Column("compose_path", sa.String(1024), nullable=True),
+        sa.Column("url", sa.String(1024), nullable=True),
+        sa.Column("port", sa.Integer(), nullable=True),
+        sa.Column("last_started_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("last_stopped_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False),
+        sa.ForeignKeyConstraint(["tool_type_id"], ["tool_types.id"]),
+        sa.ForeignKeyConstraint(["repository_id"], ["git_repositories.id"]),
+        sa.ForeignKeyConstraint(["project_id"], ["projects.id"]),
+        sa.ForeignKeyConstraint(["owner_id"], ["users.id"]),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index("idx_tool_instances_owner", "tool_instances", ["owner_id"])
+    op.create_index("idx_tool_instances_repo", "tool_instances", ["repository_id"])
+    op.create_index("idx_tool_instances_status", "tool_instances", ["status"])
+
+
+def downgrade() -> None:
+    op.drop_index("idx_tool_instances_status", table_name="tool_instances")
+    op.drop_index("idx_tool_instances_repo", table_name="tool_instances")
+    op.drop_index("idx_tool_instances_owner", table_name="tool_instances")
+    op.drop_table("tool_instances")