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/models/__init__.py

@@ -2,8 +2,9 @@ from src.models.base import Base
 from src.models.git_repository import GitRepository
 from src.models.project import Project
 from src.models.ssh_key import SSHKey
+from src.models.tool_instance import ToolInstance
 from src.models.tool_type import ToolType
 from src.models.user import User
 from src.models.user_config import UserConfig
 
-__all__ = ["Base", "GitRepository", "Project", "SSHKey", "ToolType", "User", "UserConfig"]
+__all__ = ["Base", "GitRepository", "Project", "SSHKey", "ToolInstance", "ToolType", "User", "UserConfig"]

apps/api/src/models/tool_instance.py

@@ -0,0 +1,60 @@
+import uuid
+from datetime import datetime
+from typing import TYPE_CHECKING
+
+from sqlalchemy import DateTime, ForeignKey, Integer, String
+from sqlalchemy import Uuid as UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from src.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+if TYPE_CHECKING:
+    from src.models.git_repository import GitRepository
+    from src.models.project import Project
+    from src.models.tool_type import ToolType
+    from src.models.user import User
+
+
+class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    __tablename__ = "tool_instances"
+
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    display_name: Mapped[str] = mapped_column(String(255), nullable=False)
+    tool_type_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(), ForeignKey("tool_types.id"), nullable=False
+    )
+    repository_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(), ForeignKey("git_repositories.id"), nullable=False
+    )
+    project_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(), ForeignKey("projects.id"), nullable=False
+    )
+    owner_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(), ForeignKey("users.id"), nullable=False
+    )
+    status: Mapped[str] = mapped_column(
+        String(50), nullable=False, default="pending"
+    )
+    container_id: Mapped[str | None] = mapped_column(
+        String(255), nullable=True
+    )
+    compose_path: Mapped[str | None] = mapped_column(
+        String(1024), nullable=True
+    )
+    url: Mapped[str | None] = mapped_column(
+        String(1024), nullable=True
+    )
+    port: Mapped[int | None] = mapped_column(
+        Integer, nullable=True
+    )
+    last_started_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    last_stopped_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+
+    tool_type: Mapped["ToolType"] = relationship()
+    repository: Mapped["GitRepository"] = relationship()
+    project: Mapped["Project"] = relationship()
+    owner: Mapped["User"] = relationship()