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/user.py

@@ -0,0 +1,24 @@
+from typing import TYPE_CHECKING
+
+from sqlalchemy import String
+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 import SSHKey
+    from src.models import UserConfig
+
+
+class User(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    __tablename__ = "users"
+
+    email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
+    name: Mapped[str] = mapped_column(String(255))
+    authentik_id: Mapped[str] = mapped_column(String(255), unique=True, index=True)
+    avatar_url: Mapped[str | None] = mapped_column(String(1024), nullable=True)
+
+    projects: Mapped[list["Project"]] = relationship(back_populates="owner")
+    ssh_keys: Mapped[list["SSHKey"]] = relationship(back_populates="user")
+    user_config: Mapped["UserConfig | None"] = relationship(back_populates="user", uselist=False)