feat(persistence): Prisma adapters, DI unification, Docker prod docs

[Good-Coded]

Closes #143 — Docker production deployment docs
Expanded the Docker section in README.md with a table separating variables
that must be set in production (DATABASE_URL, CORS_ORIGIN, SOROBAN_SIGNING_KEY,
contract IDs, NODE_ENV=production) from safe-to-default ones. Also adds
INTENTS_PERSISTENCE and SOLVERS_PERSISTENCE to the table.

closes #215 — Unify IntentsService with repository-injection pattern

• INTENTS_REPOSITORY Symbol DI token + IIntentsRepository interface in
  intents.repository.ts, mirroring SOLVERS_REPOSITORY exactly.
• InMemoryIntentsRepository consolidated from the deleted
  in-memory-intents.repository.ts — no separate file, one canonical adapter.
• IntentsService injects IIntentsRepository; private Map removed.
  Idempotency cache and audit log remain in-service (non-durable concerns).
• IntentsModule uses useFactory to select adapter via INTENTS_PERSISTENCE env var.

closes #214 — PrismaIntentsRepository with atomic conditional updates

• src/intents/prisma-intents.repository.ts
• acceptIfOpen: single prisma.intent.updateMany({ where: { intentId, state:'open' }})
  — 0 rows = race lost, no separate read-then-write.
• fillIfAccepted: updateMany guards on state='accepted' AND solver atomically.
• Safe for horizontal scaling. Selected via INTENTS_PERSISTENCE=prisma.

closes #216 — PrismaSolversRepository

• src/solvers/prisma-solvers.repository.ts
• save() uses upsert keyed on unique address — register/deactivate/reactivate/
  recordFailedFill all work unmodified.
• bondAmount/totalVolume stay strings; never coerced through JS number.
• Selected via SOLVERS_PERSISTENCE=prisma (default: memory).

Docs

• docs/architecture/onchain-settlement.md: added Persistence layer section.
• CONTRIBUTING.md: updated Module and file structure with canonical pattern.

Tests

• All specs updated for async service methods (await).
• intents.service.spec.ts rebuilt around Test.createTestingModule + DI token.
• Pre-existing failures in http-exception.filter.spec, logging.interceptor.spec,
  soroban.controller.spec (18 tests on main before this PR) are not introduced
  by this change — verified by stash test.

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