feat(backend): add resilient FX provider caching and stale-data semantics

[arisu6804]

Problem

Provider outages can block transfers or cause stale exchange rates to be mistaken for current quotes.

Objective

Deliver a production-quality improvement to FX adapters, quotes, and transfer pricing that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Add bounded cache TTL, provider fallback, freshness metadata, quote versioning, and explicit stale/error policies; prevent stampedes.

Acceptance criteria

• Stale quotes are visible and cannot be used outside policy; fallback behavior is deterministic; quote identity is bound to transfer creation.

Required validation

• Provider failure, expiry, fallback, stampede, and quote-binding tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.