Add distribution and beneficiary assignment search

[FroDev-CR]

• Add SearchService.searchDistributions() / searchAssignments(): filtering, pagination and sorting for Distribution and BeneficiaryAssignment records, reusing the existing search pagination/sort-validation helpers.
• Distribution filters: id/blockchainTxHash/transactionRef, campaign (id or name), beneficiary (id or name), status, method, location (beneficiary country/city), distributedBy, date range, amount range.
• Assignment filters: id, campaign (id or name), beneficiary (id or name), needsCategory, location, priority range, date range. Note: the BeneficiaryAssignment model has no status/completedAt field, so status filtering isn't available for this endpoint (documented in API.md).
• New GET /search/distributions and GET /search/assignments endpoints (Admin/Verifier only), plus entityType=distribution|assignment support in the existing GET /search/advanced endpoint.
• Add zod validation schemas (distributionSearchSchema, assignmentSearchSchema) matching the existing beneficiarySearchSchema conventions.
• Add missing indexes on BeneficiaryAssignment (assignedAt, priority) via migration; Distribution already had campaignId/beneficiaryId/status/ distributedAt indexed.
• Unit tests covering where-clause construction, sorting, pagination, and advancedSearch entity routing for both new endpoints.
• Document both endpoints (query params, examples, response shape) in API.md.