feat: multi-session terminal backend core (PR 1)

- Add TerminalSessionModel DB table with instance_id FK, name, status,
  created_at, last_activity_at, closed_at columns
- Add Alembic migration for terminal_sessions table
- Refactor TerminalManager to use composite key (instance_id, session_id)
  supporting up to 5 concurrent sessions per instance
- Add create_session, get_session, get_sessions_for_instance, close_session
- Preserve get_or_create_session for backward compatibility (default session)
- Fix attach_websocket to only close sockets within same session
- Add name (auto-generated 'Session N') and status tracking to TerminalSession
- Add 7 unit tests for multi-session logic

Quality gates: pytest (7 new passed, 174 total passed, 51 pre-existing failures)

apps/api/src/models/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,
+    )