feat: implement Phase 2 - Multiple active meetings per room with grace period

This commit adds support for multiple concurrent meetings per room, implementing
grace period logic and improved meeting lifecycle management for calendar integration.

## Database Changes
- Remove unique constraint preventing multiple active meetings per room
- Add last_participant_left_at field to track when meeting becomes empty
- Add grace_period_minutes field (default: 15) for configurable grace period

## Meeting Controller Enhancements
- Add get_all_active_for_room() to retrieve all active meetings for a room
- Add get_active_by_calendar_event() to find meetings by calendar event ID
- Maintain backward compatibility with existing get_active() method

## New API Endpoints
- GET /rooms/{room_name}/meetings/active - List all active meetings
- POST /rooms/{room_name}/meetings/{meeting_id}/join - Join specific meeting

## Meeting Lifecycle Improvements
- 15-minute grace period after last participant leaves
- Automatic reactivation when participant rejoins during grace period
- Force close calendar meetings 30 minutes after scheduled end time
- Update process_meetings task to handle multiple active meetings

## Whereby Integration
- Clear grace period when participants join via webhook events
- Track participant count for grace period management

## Testing
- Add comprehensive tests for multiple active meetings
- Test grace period behavior and participant rejoin scenarios
- Test calendar meeting force closure logic
- All 5 new tests passing

This enables proper calendar integration with overlapping meetings while
preventing accidental meeting closures through the grace period mechanism.

server/migrations/versions/6025e9b2bef2_remove_one_active_meeting_per_room_.py

@@ -0,0 +1,53 @@
+"""remove_one_active_meeting_per_room_constraint
+
+Revision ID: 6025e9b2bef2
+Revises: 9f5c78d352d6
+Create Date: 2025-08-18 18:45:44.418392
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "6025e9b2bef2"
+down_revision: Union[str, None] = "9f5c78d352d6"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Remove the unique constraint that prevents multiple active meetings per room
+    # This is needed to support calendar integration with overlapping meetings
+    # Check if index exists before trying to drop it
+    from alembic import context
+
+    if context.get_context().dialect.name == "postgresql":
+        conn = op.get_bind()
+        result = conn.execute(
+            sa.text(
+                "SELECT 1 FROM pg_indexes WHERE indexname = 'idx_one_active_meeting_per_room'"
+            )
+        )
+        if result.fetchone():
+            op.drop_index("idx_one_active_meeting_per_room", table_name="meeting")
+    else:
+        # For SQLite, just try to drop it
+        try:
+            op.drop_index("idx_one_active_meeting_per_room", table_name="meeting")
+        except:
+            pass
+
+
+def downgrade() -> None:
+    # Restore the unique constraint
+    op.create_index(
+        "idx_one_active_meeting_per_room",
+        "meeting",
+        ["room_id"],
+        unique=True,
+        postgresql_where=sa.text("is_active = true"),
+        sqlite_where=sa.text("is_active = 1"),
+    )

server/migrations/versions/d4a1c446458c_add_grace_period_fields_to_meeting.py

@@ -0,0 +1,34 @@
+"""add_grace_period_fields_to_meeting
+
+Revision ID: d4a1c446458c
+Revises: 6025e9b2bef2
+Create Date: 2025-08-18 18:50:37.768052
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "d4a1c446458c"
+down_revision: Union[str, None] = "6025e9b2bef2"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add fields to track when participants left for grace period logic
+    op.add_column(
+        "meeting", sa.Column("last_participant_left_at", sa.DateTime(timezone=True))
+    )
+    op.add_column(
+        "meeting",
+        sa.Column("grace_period_minutes", sa.Integer, server_default=sa.text("15")),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("meeting", "grace_period_minutes")
+    op.drop_column("meeting", "last_participant_left_at")