Implement § 61 NV Bühne non-renewal deadline calculation with tiered
protection (standard 31.10., extended 31.07. for 15+ years, special
protection for over-55), tariff-based compensation calculation with
Gagenklassen and Dienstalterszulage, and Spielzeit seasonal logic
(1.8.–31.7. with Probenzeit). Includes DB schema (contracts,
compensationRules, nonRenewalDeadlines), migration, and three API
endpoints under /api/nv-buehne/.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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