feat(stellar): add Stellar account fields and Privy user linking flow

[libby-coder]

Summary

• Fix duplicate stellarPublicKey field in models/User.ts — the schema had two definitions of the same field (one with unique: true, one with index: true). Merged into a single correct definition with unique, sparse, trim, and index all set.
• Add all four Stellar user fields as optional schema additions: stellarPublicKey, stellarAccountType, stellarLinkedAt, stellarLastSyncedAt. All fields default gracefully so existing users are unaffected with no migration needed.
• Set stellarLinkedAt on first link in POST /api/auth/stellar/link — the timestamp is recorded when a key is first attached and is never overwritten on subsequent re-links.
• Extend route tests with two new cases: verifying stellarLinkedAt is set on first link and is preserved (not reset) on re-link.
• Add docs/auth-stellar.md — documents how Privy auth and Stellar account linking fit together, the security properties of each layer (public-key-only storage, StrKey checksum validation, unique index + 409 race protection), relevant environment variables, and contributor setup instructions.

Files changed

File	Change
models/User.ts	Remove duplicate stellarPublicKey; keep single definition with unique, sparse, trim, index
app/api/auth/stellar/link/route.ts	Set stellarLinkedAt on first link only
app/api/auth/stellar/link/route.test.ts	Add tests for stellarLinkedAt first-link and re-link preservation
docs/auth-stellar.md	New — Privy + Stellar architecture, security properties, env vars, contributor guide

Acceptance criteria checklist

• User model supports optional Stellar account fields
• Existing users continue to work (all fields optional, no migration)
• Stellar public key validation is applied before saving (StrKey + CRC16 checksum)
• Privy remains the authentication layer
• No private key is stored or exposed
• stellarLinkedAt is recorded on first link
• Documentation added explaining how Privy auth and Stellar accounts fit together

Closes #27

[Obiajulu-gif]

@libby-coder great work sir, can you fix the lint error

[libby-coder]

@libby-coder great work sir, can you fix the lint error

The typecheck CI failure is not caused by changes in this PR. The four files I modified are:

models/User.ts
app/api/auth/stellar/link/route.ts
app/api/auth/stellar/link/route.test.ts
docs/auth-stellar.md

Every error reported by tsc --noEmit is in unrelated files (app/dashboard/admin/loans/page.tsx, components/ui/chart.tsx, components/providers.tsx, components/dashboard/advanced-analytics.tsx, etc.) that this PR does not touch. These errors exist on main and pre-date this branch — verifiable with git diff main...HEAD --name-only.