[#144] Add fee-bump transaction support to wrap user transactions paying the base fee

[BigNathan1]

Backlog reference: #144 · stellar

Summary

Add fee-bump transaction support to wrap user transactions paying the base fee

Context

The backend/src/contracts/ Stellar SDK wrapper layer (stellar.ts, assets.ts, trustlines.ts, transactions.ts) that every route handler calls into. This is the one place Horizon interaction logic should live — route handlers stay thin and testable.

Foundation work to reach a working single-community MVP: infrastructure, schema, core CRUD APIs, the Stellar contract layer, the base frontend, wallet auth, tests, CI, and a hardened security baseline. Every later phase assumes this phase is solid.

Acceptance Criteria

• Add fee-bump transaction support to wrap user transactions paying the base fee — implemented as described above.
• The Horizon/Stellar interaction is verified against testnet, not just mocked.
• Stellar/Horizon error codes are mapped to clear, actionable messages rather than surfaced raw.
• Unit tests mock Horizon so the suite stays fast and deterministic.

Definition of Done

• Implemented on a branch and opened as a PR referencing this issue (Closes #144 won't auto-link since this is a repo-internal reference — mention the issue number GitHub assigned instead).
• CI is green (lint, typecheck, tests, build).
• Reviewed against CONTRIBUTING.md conventions (Conventional Commits, code style).
• CHANGELOG.md updated under [Unreleased] if the change is user- or API-visible.
• Merged to main.

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