api-contracts: add HIGH and MEDIUM priority request validation schemas
- Completed Action 1.2.1.4: Added 6 missing request schemas
- HIGH priority (2FA security): verify2FARequestSchema, disable2FARequestSchema
- MEDIUM priority (core features): batchDeleteTracksRequestSchema, initiateChunkedUploadRequestSchema, completeChunkedUploadRequestSchema, uploadChunkRequestSchema
- All schemas validated against Swagger spec definitions
- Note: /auth/2fa/setup has no request body (generates secret/QR code)
- Note: /tracks/chunk uses formData (multipart), chunk file validated separately
- Type safety verified, no regressions
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