fix(docker): specify pnpm@9 to match lockfile version

[GMNAPI]

Problem

Railway deployment failing with:

ERR_PNPM_LOCKFILE_BREAKING_CHANGE  Lockfile /app/pnpm-lock.yaml not compatible with current pnpm

Root Cause

• pnpm-lock.yaml uses lockfileVersion: '9.0' (requires pnpm 9.x)
• Dockerfile runs npm install -g pnpm without version specification
• Railway installs whatever latest/default pnpm is available (could be 8.x or 10.x)
• Version mismatch → lockfile incompatible → build fails

Solution

Explicitly specify pnpm@9 in both Dockerfile stages:

# Before
RUN npm install -g pnpm

# After  
RUN npm install -g pnpm@9

This ensures the pnpm version matches the lockfile version requirement.

Testing

After merge, Railway should successfully:

1. Install pnpm 9.x
2. Read pnpm-lock.yaml without errors
3. Install dependencies
4. Deploy successfully

Related

• Blocking deployment of PR feat(api): add debug endpoints for prod/local diagnosis #17 (debug endpoints)
• Affects all future deployments until fixed

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