feat(backend): enhance distributed concurrency control and locking for SEP-0001 Stellar Info Generator

[dragoncode-01]

…

• Add sep0001-toml-coordinator: per-process single-flight, Redis SET NX PX lock with owner tokens and compare-and-delete release, a shared TOML store with SHA-256 integrity checks, and generation fencing so a regeneration that races an invalidation cannot publish stale content
• Fail open to direct generation when Redis is unavailable, errors, or is the no-op fallback client
• Route: strict UUID validation for merchant_id, lower-case normalisation, strong ETag / If-None-Match (304)
• Invalidate stellar.toml on merchant branding updates (route and service)
• Document design, security notes and SEP1_TOML_* configuration
• Add coordinator and route tests (41 new tests)

Closes #1460

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