Remove grace period logic and improve meeting deactivation

- Removed grace_period_minutes and last_participant_left_at fields
- Simplified deactivation logic based on actual usage patterns:
  * Active sessions: Keep meeting active regardless of scheduled time
  * Calendar meetings: Wait until scheduled end if unused, deactivate immediately once used and empty
  * On-the-fly meetings: Deactivate immediately when empty
- Created migration to drop unused database columns
- Updated tests to remove grace period test cases

server/migrations/versions/dc035ff72fd5_remove_grace_period_fields.py

@@ -0,0 +1,43 @@
+"""remove_grace_period_fields
+
+Revision ID: dc035ff72fd5
+Revises: d8e204bbf615
+Create Date: 2025-09-11 10:36:45.197588
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "dc035ff72fd5"
+down_revision: Union[str, None] = "d8e204bbf615"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Remove grace period columns from meeting table
+    op.drop_column("meeting", "last_participant_left_at")
+    op.drop_column("meeting", "grace_period_minutes")
+
+
+def downgrade() -> None:
+    # Add back grace period columns to meeting table
+    op.add_column(
+        "meeting",
+        sa.Column(
+            "last_participant_left_at", sa.DateTime(timezone=True), nullable=True
+        ),
+    )
+    op.add_column(
+        "meeting",
+        sa.Column(
+            "grace_period_minutes",
+            sa.Integer(),
+            server_default=sa.text("15"),
+            nullable=True,
+        ),
+    )