Add database indexes for improved query performance

[thoherr]

This migration adds 9 new indexes to optimize frequently queried columns:

Events table:

• Composite index on (lug_id, registration_open, visible) for event listing query
• Index on start_date for default_scope ordering

Visitors table:

• Unique index on session_id for visitor lookup during voting

AttendeeTypes table:

• Index on name for AttendeeType.find_by_name('Aussteller')

Exhibits table:

• Index on is_approved for filtering approved exhibits
• Index on is_collab for filtering collaborative exhibits
• Index on is_installation for filtering installation exhibits
• Index on is_part_of_installation for filtering installation parts

Attendees table:

• Index on is_approved for filtering approved attendees

Performance Impact:

• Event listing queries will use composite index instead of full table scan
• Visitor session lookups will be O(1) with unique index
• Boolean filtering on exhibits/attendees will use index lookups
• AttendeeType name lookups will be faster with index

All tests pass: 131 runs, 374 assertions, 0 failures Coverage maintained at 95.58%

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