feat: notification center backend core (PR-1)

- Add notifications table with Alembic migration
- Notification model with user-scoped indexing and partial index on unread
- NotificationService singleton with create/list/count/mark-read/dismiss
- FastAPI router: GET /notifications, GET /unread, PATCH /{id}/read,
  POST /mark-all-read, DELETE /{id}
- Mute categories filtering from UserConfig
- 13 unit tests for NotificationService
- 10 integration tests for API endpoints
- Updated test_models.py with new table registration

Quality gates: pytest 23 new passed, ruff clean

apps/api/src/models/__init__.py

@@ -3,6 +3,7 @@ from src.models.config_profile import ConfigProfile, ConfigProfileInclude
 from src.models.git_repository import GitRepository
 from src.models.health_check import HealthCheck
 from src.models.instance_event import InstanceEvent
+from src.models.notification import Notification
 from src.models.project import Project
 from src.models.ssh_key import SSHKey
 from src.models.terminal_session import TerminalSessionModel
@@ -19,6 +20,7 @@ __all__ = [
     "GitRepository",
     "HealthCheck",
     "InstanceEvent",
+    "Notification",
     "Project",
     "SSHKey",
     "TerminalSessionModel",

apps/api/src/models/notification.py

@@ -0,0 +1,43 @@
+"""Notification SQLAlchemy model."""
+
+from datetime import datetime
+from typing import Any
+import uuid
+
+from sqlalchemy import DateTime, ForeignKey, JSON, String, Text
+from sqlalchemy import Uuid as UUID
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.sql import func
+
+from src.models.base import Base, UUIDPrimaryKeyMixin
+
+
+class Notification(UUIDPrimaryKeyMixin, Base):
+    __tablename__ = "notifications"
+
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    category: Mapped[str] = mapped_column(String(32), nullable=False)
+    severity: Mapped[str] = mapped_column(String(16), nullable=False)
+    title: Mapped[str] = mapped_column(String(255), nullable=False)
+    message: Mapped[str | None] = mapped_column(Text, nullable=True)
+    source_type: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    source_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), nullable=True
+    )
+    notification_metadata: Mapped[dict[str, Any]] = mapped_column(
+        "metadata", JSON, nullable=False, default=dict
+    )
+    read_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True, index=True
+    )
+    dismissed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False, index=True
+    )