Backend: Persist wallet alias mappings for cross-service identity resolution

[Junirezz]

Context

walletAliasService.ts resolves wallet identity links across aliases and sources. referralService.ts already depends on canonical wallet resolution via this service.

Problem / Gap

Alias mappings are stored only in memory. Restarts wipe alias registrations, breaking referral attribution and any admin workflows that rely on persisted identity links.

Proposed approach

• Add Prisma tables for canonical wallet IDs, alias rows, and source metadata.
• Implement upsert/merge semantics matching current in-memory behavior.
• Seed service from DB on startup; keep in-memory cache for hot reads.
• Add admin endpoints to list and delete alias groups if not already exposed.

Acceptance criteria

• Registered aliases survive restart and resolve to the same canonical ID.
• Merge behavior when the same alias maps to different canonical IDs matches current tests.
• Referral flows continue to resolve wallets via persisted aliases.
• Migration path documented for environments with existing in-memory-only data.

Files/areas affected

• backend/src/walletAliasService.ts
• backend/src/walletAliasEndpoints.ts
• backend/src/referralService.ts
• backend/prisma/schema.prisma