chore(ci): add yarn lint:fix script for ESLint auto-fixes

[Harsh23Kashyap]

Problem

yarn lint only reports ESLint issues; it doesn't auto-fix them. To apply ESLint's safe auto-fixes today, contributors have to run eslint --fix per-workspace manually (5 invocations across web, backend, db, schemas, shared).

Motivation

CONTRIBUTING.md documents running lint per-workspace. There is no top-level lint:fix shortcut. Adding one matches the existing yarn lint / yarn test / yarn typecheck delegation pattern.

Proposed solution

• Root package.json: add "lint:fix": "yarn workspaces foreach --all --topological run lint:fix".
• Each of the 5 workspace package.jsons: add "lint:fix": "cross-env SKIP_ENV_VALIDATION=1 eslint --fix ." (matching the existing lint shape).

Scope

6 files, +6 lines, no source code touched. Per-workspace invocation delegates via workspaces foreach --all --topological.

Acceptance criteria

• yarn lint:fix from repo root succeeds end-to-end.
• Subsequent yarn lint shows no fixable issues.
• No new dependencies added.
• Document the new script in CONTRIBUTING.md.

Backward compatibility

Pure additive tooling. Existing yarn lint, yarn test, yarn build, yarn typecheck unchanged.