fix: Address review feedback on onboarding cleanup

[absarasw]

Adding changes for review comment at #2283 (comment)

• Move PLG_OPPORTUNITY_TYPES into shared src/controllers/plg/plg-constants.js
  so plg-onboarding.js (displacement check) and plg-onboarding-cleanup.js
  (status reset + FixEntity deletion) cannot drift.
• Promote the missing-siteId guard log from info to warn (caller-bug signal).
• Drop unused setStatus stub from createMockSuggestion test helper.
• Document the discarded return value and add an authorization-contract
  note to the cleanupPlgSiteSuggestionsAndFixes JSDoc.
• Emit a structured "PLG cleanup summary [found=true|false]" line so ops
  can grep / aggregate runs that actually found stale rows.

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