feat: add POST /api/contracts/deploy endpoint (#33)

[patopatrish]

Summary

• DB migration (scripts/006-contracts.sql): adds a contracts table with UNIQUE(job_id) to enforce one contract per job, and idempotently adds a contract_id FK column to the existing milestones table.
• Soroban service (lib/soroban/deploy.ts): introduces deploySorobanEscrow() with a deterministic stub and a detailed TODO block mapping out the real Stellar SDK steps (WASM upload → contract install → transaction polling).
• DB store (lib/contracts/store.ts): typed helpers — getJobById, getExistingContract, getUserById, createContract, createMilestones, linkJobToContract.
• API route (app/api/contracts/deploy/route.ts): POST /api/contracts/deploy protected by the existing withAuth() middleware. Validates all inputs, verifies job ownership, guards against duplicate deployments, calls the Soroban service, persists the contract + milestones, and back-patches jobs.escrow_contract_id.

Error codes

Code | HTTP | Meaning -- | -- | -- INVALID_JOB_ID / INVALID_FREELANCER_ID / INVALID_TOTAL_AMOUNT | 400 | Missing or malformed fields INVALID_MILESTONES | 400 | Milestone array fails per-element validation AUTH_REQUIRED | 401 | Missing / expired access token USER_NOT_FOUND | 401 | Wallet has no platform account FORBIDDEN | 403 | Caller is not the job's client JOB_NOT_FOUND / FREELANCER_NOT_FOUND | 404 | Resource doesn't exist CONTRACT_ALREADY_EXISTS / JOB_NOT_DEPLOYABLE | 409 | Business-rule conflicts DEPLOYMENT_FAILED | 500 | Soroban deployment threw DB_ERROR | 500 | Persistence failure

Test plan

•  Run scripts/006-contracts.sql against your Neon DB
•  POST /api/contracts/deploy with a valid JWT, existing jobId owned by the caller, a valid freelancerId, and totalAmount — expect 201 with contractAddress and txHash
•  Repeat the same request — expect 409 CONTRACT_ALREADY_EXISTS
•  Send without auth header — expect 401 AUTH_REQUIRED
•  Send with a jobId belonging to a different client — expect 403 FORBIDDEN
•  Send missing required fields — expect 400 with the relevant code
•  Swap deploySorobanEscrow stub for real SDK calls when Soroban integration is ready

Closes #33

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