fix: optimistic locking for docs editor (prevent edit conflicts)

[AngryJay91]

Summary

Closes #15

Adds version-based optimistic locking to the docs editor to prevent silent data loss when multiple users edit the same document simultaneously.

Changes

scaffold/pm-api/sql/009-doc-versioning.sql (new)

• Adds version INTEGER DEFAULT 1 column to the docs table

scaffold/pm-api/src/routes/v2-docs.ts

• GET /:id: now returns version field in doc response
• PUT /:id: accepts optional version in request body
  • New doc (INSERT): sets version = 1
  • Existing doc (UPDATE): checks WHERE version = ?, increments on success
  • Returns 409 Conflict with code: 'VERSION_CONFLICT' if version mismatch
  • Legacy clients (no version sent): graceful degradation, skips conflict check

scaffold/spec-site/src/pages/DocsEditor.vue

• Reads version from GET response and stores locally
• Sends version with every PUT save request
• On 409: shows amber warning banner ("Someone else edited this document...")
• On success: updates local version from server response

Behavior

User A loads doc (version=3)
User B loads doc (version=3) 
User B saves → version becomes 4
User A saves with version=3 → WHERE version=3 matches 0 rows → 409
User A sees: "Someone else edited this document while you were working..."