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/user/ssh_key.py

@@ -0,0 +1,25 @@
+import uuid
+from typing import TYPE_CHECKING
+
+from sqlalchemy import ForeignKey, String, Text
+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.project import Project
+    from src.models.user import User
+
+
+class SSHKey(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    __tablename__ = "ssh_keys"
+
+    name: Mapped[str] = mapped_column(String(255))
+    public_key: Mapped[str] = mapped_column(Text)
+    private_key_encrypted: Mapped[str] = mapped_column(Text)
+    user_id: Mapped[uuid.UUID] = mapped_column(UUID(), ForeignKey("users.id"), nullable=False)
+    project_id: Mapped[uuid.UUID | None] = mapped_column(UUID(), ForeignKey("projects.id"), nullable=True)
+
+    user: Mapped["User"] = relationship(back_populates="ssh_keys")
+    project: Mapped["Project | None"] = relationship(back_populates="ssh_keys", foreign_keys=[project_id])