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/src/services/docker.py

@@ -0,0 +1,171 @@
+"""Docker service for managing tool instances."""
+
+import os
+import subprocess
+from pathlib import Path
+from typing import Any
+
+
+def render_compose_template(template: str, variables: dict[str, Any]) -> str:
+    """Render a Docker Compose template with variable substitution.
+
+    Args:
+        template: The compose template string
+        variables: Dictionary of variable names to values
+
+    Returns:
+        Rendered compose file content
+    """
+    result = template
+    for key, value in variables.items():
+        placeholder = f"{{{{{key}}}}}"
+        result = result.replace(placeholder, str(value))
+    return result
+
+
+def ensure_instance_directory(instance_id: str, base_path: str = "data/instances") -> str:
+    """Create and return the instance directory path.
+
+    Args:
+        instance_id: Unique instance identifier
+        base_path: Base directory for all instances
+
+    Returns:
+        Absolute path to instance directory
+    """
+    instance_dir = Path(base_path) / instance_id
+    instance_dir.mkdir(parents=True, exist_ok=True)
+    return str(instance_dir.absolute())
+
+
+def write_compose_file(instance_dir: str, content: str) -> str:
+    """Write the rendered compose file to the instance directory.
+
+    Args:
+        instance_dir: Path to instance directory
+        content: Rendered compose content
+
+    Returns:
+        Path to the compose file
+    """
+    compose_path = Path(instance_dir) / "docker-compose.yml"
+    compose_path.write_text(content)
+    return str(compose_path)
+
+
+def execute_compose_command(
+    compose_path: str, action: str, timeout: int = 60
+) -> tuple[int, str, str]:
+    """Execute a docker compose command.
+
+    Args:
+        compose_path: Path to docker-compose.yml
+        action: The compose action (up, down, start, stop, restart)
+        timeout: Command timeout in seconds
+
+    Returns:
+        Tuple of (returncode, stdout, stderr)
+    """
+    instance_dir = Path(compose_path).parent
+
+    cmd = ["docker", "compose", "-f", compose_path]
+
+    if action == "up":
+        cmd.extend(["up", "-d"])
+    elif action == "down":
+        cmd.extend(["down", "-v"])
+    elif action in ("start", "stop", "restart"):
+        cmd.append(action)
+    else:
+        raise ValueError(f"Unknown compose action: {action}")
+
+    result = subprocess.run(
+        cmd,
+        cwd=str(instance_dir),
+        capture_output=True,
+        text=True,
+        timeout=timeout,
+    )
+
+    return result.returncode, result.stdout, result.stderr
+
+
+def get_container_id(instance_name: str) -> str | None:
+    """Get the container ID for a compose service.
+
+    Args:
+        instance_name: The service name in compose
+
+    Returns:
+        Container ID or None if not found
+    """
+    result = subprocess.run(
+        ["docker", "ps", "-q", "--filter", f"name={instance_name}"],
+        capture_output=True,
+        text=True,
+    )
+
+    if result.returncode == 0 and result.stdout.strip():
+        return result.stdout.strip().split("\n")[0]
+    return None
+
+
+def get_container_status(container_id: str) -> str:
+    """Get the status of a Docker container.
+
+    Args:
+        container_id: Docker container ID
+
+    Returns:
+        Container status string (running, exited, etc.)
+    """
+    result = subprocess.run(
+        ["docker", "inspect", "-f", "{{.State.Status}}", container_id],
+        capture_output=True,
+        text=True,
+    )
+
+    if result.returncode == 0:
+        return result.stdout.strip()
+    return "unknown"
+
+
+def get_container_logs(container_id: str, tail: int = 100) -> str:
+    """Get the logs of a Docker container.
+
+    Args:
+        container_id: Docker container ID
+        tail: Number of lines to return
+
+    Returns:
+        Container logs
+    """
+    result = subprocess.run(
+        ["docker", "logs", "--tail", str(tail), container_id],
+        capture_output=True,
+        text=True,
+    )
+
+    if result.returncode == 0:
+        return result.stdout
+    return f"Failed to get logs: {result.stderr}"
+
+
+def find_free_port(start: int = 10000, end: int = 20000) -> int:
+    """Find a free TCP port in the given range.
+
+    Args:
+        start: Start of port range
+        end: End of port range
+
+    Returns:
+        Free port number
+    """
+    import socket
+
+    for port in range(start, end):
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+            if s.connect_ex(("localhost", port)) != 0:
+                return port
+
+    raise RuntimeError(f"No free port found in range {start}-{end}")