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/alembic/versions/2026_05_28_add_terminal_sessions_table.py

@@ -0,0 +1,61 @@
+"""add terminal_sessions table
+
+Revision ID: 2026_05_28_add_terminal_sessions
+Revises: 20260527_160017_add_pi_agent
+Create Date: 2026-05-28
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = "2026_05_28_add_terminal_sessions"
+down_revision: Union[str, None] = "20260527_160017_add_pi_agent"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "terminal_sessions",
+        sa.Column("id", sa.UUID(), nullable=False),
+        sa.Column("instance_id", sa.UUID(), nullable=False),
+        sa.Column("name", sa.String(length=255), nullable=True),
+        sa.Column("status", sa.String(length=50), nullable=False),
+        sa.Column("last_activity_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("closed_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            nullable=False,
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            onupdate=sa.text("now()"),
+            nullable=False,
+        ),
+        sa.ForeignKeyConstraint(
+            ["instance_id"], ["tool_instances.id"], ondelete="CASCADE"
+        ),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index(
+        op.f("ix_terminal_sessions_instance_id"),
+        "terminal_sessions",
+        ["instance_id"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(
+        op.f("ix_terminal_sessions_instance_id"),
+        table_name="terminal_sessions",
+    )
+    op.drop_table("terminal_sessions")