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/system/terminal_session.py

@@ -0,0 +1,37 @@
+"""Terminal session database model."""
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String
+from sqlalchemy import Uuid as UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from src.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class TerminalSessionModel(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """Database model for terminal session metadata."""
+
+    __tablename__ = "terminal_sessions"
+
+    instance_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(),
+        ForeignKey("tool_instances.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    name: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    status: Mapped[str] = mapped_column(
+        String(50),
+        nullable=False,
+        default="active",
+    )
+    last_activity_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )
+    closed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )