fix(server): resolve DATABASE_URL via ConfigService with robust production guard (closes #172)

[s6pa1rta3n-lab]

Description

Addresses #172 by ensuring DATABASE_URL is resolved exclusively through ConfigService and validating that local file:-scheme URLs are blocked in production environments.

Payout Stipulations Checklist

• DatabaseModule resolves DATABASE_URL exclusively through ConfigService, with no direct process.env read remaining.
• A production-mode guard against the local-file default exists and prevents initializing PrismaClient against file: in production.
• Unit tests cover assertDatabaseUrlAllowed validation across environments (production vs. development/test).
• All server CI suites (lint, format check, typecheck, unit tests, build) pass with zero errors.

Changes

• Updated DatabaseModule to inject ConfigService for DATABASE_URL resolution.
• Hardened assertDatabaseUrlAllowed to guard against empty/undefined URLs and reject file: URLs in production.
• Added comprehensive unit tests in server/src/database/database.module.spec.ts.

Verification

• Ran npm run lint -> Passed
• Ran npm run format:check -> Passed
• Ran npm run typecheck -> Passed
• Ran npm test -> 10/10 test suites passed (123 total tests)
• Ran npm run build -> Passed

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC