Fix Stellar balance error swallowing, JWT secret fallback, and admin auth route split

[memplethee-lab]

Summary

Three critical fixes addressing security, data integrity, and authentication availability.

#13 - Stop swallowing Horizon errors in StellarProvider.getBalance

Problem: Bare catch in StellarProvider.getBalance returned 0 for any failure (DNS, 429, 5xx, timeouts). WalletsService.updateBalance then persisted that 0, corrupting real balances and causing legitimate payments to be rejected by the "insufficient balance" check.

Fix:

• 404 (unfunded account) still resolves to 0
• All other errors throw BlockchainProviderUnavailableError with chain, address, and original cause
• Errors logged via NestJS Logger (no raw console.*)
• WalletsService.updateBalance re-throws so stored balance remains untouched
• Added BlockchainProviderUnavailableError class exported from BlockchainService

#12 - Remove hardcoded JWT secret fallback in JwtStrategy

Problem: JwtStrategy fell back to a publicly known secret (default_jwt_secret_key_change_in_production) if JWT_SECRET was unset. .env.example and docker-compose.yml shipped the placeholder as a usable value, so a misconfigured deployment silently booted with a compromised secret.

Fix:

• Added validateJwtSecret() that throws at startup if secret is missing, empty, or a known placeholder
• Applied in both JwtStrategy and AdminModule/JwtModule.register
• Removed the literal fallback string from all source files
• .env.example now shows JWT_SECRET= (empty, must be set)
• docker-compose.yml uses ${JWT_SECRET} without default
• Tests cover missing, placeholder, and valid secrets

#11 - Split public admin auth routes from protected admin routes

Problem: @UseGuards(AdminGuard, RolesGuard) on the whole AdminController ran before @Roles() could opt out. AdminGuard unconditionally required a bearer token, so POST /admin/login, POST /admin/otp/generate, POST /admin/otp/verify all returned 401 — admins could never obtain their first token.

Fix:

• New AdminAuthController at /admin/auth with no guards (login, OTP generate, OTP verify)
• AdminController now only contains protected routes under /admin
• Both controllers registered in AdminModule
• Fixed AdminGuard to not mask its own UnauthorizedException("Admin access required") as generic "Invalid or expired token" by restructuring the try/catch
• Updated docs/api-guide.md with new /admin/auth/* routes
• Tests cover: public routes without token, protected routes without token, protected routes with non-admin token

Testing

Added test files:

• src/modules/blockchain/__tests__/blockchain.service.spec.ts - success, unfunded (404), network failure, 5xx, 429, timeout
• src/modules/auth/__tests__/jwt.strategy.spec.ts - missing secret, placeholder secret, valid secret
• src/modules/admin/__tests__/admin.guard.spec.ts - missing token, invalid token, non-admin payload, valid admin, error message not masked
• src/modules/wallets/__tests__/wallets.service.spec.ts - balance update success, provider error leaves balance untouched, wallet not found

Notes

• No npm install, CI, or tests run locally per instructions
• All three issues addressed in a single PR as they share the contributor (Memplethee-lab)

Closes #13
Closes #12
Closes #11