refactor: reorganize models into subpackages

Move models into domain subpackages (max 4 files each):
- models/tool/ — tool_type, tool_instance, tool_definition_manifest
- models/config/ — config_profile
- models/user/ — user, user_config, ssh_key
- models/project/ — project, git_repository, workspace
- models/system/ — health_check, notification, instance_event, terminal_session

models/__init__.py continues to re-export all symbols, so consumers
using 'from src.models import X' are unaffected. Updated direct file
imports across the backend to use the new paths.

Quality gates: py_compile passed, ruff passed.

apps/api/src/models/tool/tool_instance.py

@@ -0,0 +1,73 @@
+import uuid
+from datetime import datetime
+from typing import TYPE_CHECKING
+
+from sqlalchemy import DateTime, ForeignKey, Integer, JSON, 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 import ConfigProfile
+    from src.models import GitRepository
+    from src.models.project import Project
+    from src.models import ToolType
+    from src.models.user import User
+    from src.models import Workspace
+
+
+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)
+    container_name: 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)
+    public_url: Mapped[str | None] = mapped_column(String(1024), nullable=True)
+    tunnel_id: Mapped[str | None] = mapped_column(String(255), 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
+    )
+    manifest_compiled_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    image_tag: Mapped[str | None] = mapped_column(String(256), nullable=True)
+    probe_result: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    clone_mode: Mapped[str] = mapped_column(String(20), nullable=False, default="mount")
+    branch: Mapped[str | None] = mapped_column(
+        String(255), nullable=True, default="main"
+    )
+    selected_config_profile_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(), ForeignKey("config_profiles.id", ondelete="SET NULL"), nullable=True
+    )
+    ssh_key_ids: Mapped[list[str] | None] = mapped_column(JSON, nullable=True)
+    workspace_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(), ForeignKey("workspaces.id", ondelete="SET NULL"), nullable=True
+    )
+
+    tool_type: Mapped["ToolType"] = relationship()
+    workspace: Mapped["Workspace | None"] = relationship()
+    repository: Mapped["GitRepository"] = relationship()
+    project: Mapped["Project"] = relationship()
+    owner: Mapped["User"] = relationship()
+    selected_config_profile: Mapped["ConfigProfile | None"] = relationship()