feat(services): add optimistic price updates with rollback

[woahwhattheheck]

Closes #624

What changed

• add a service-scoped optimistic mutation ledger keyed by serviceId with monotonic revisions
• publish price edits immediately to services dashboard/detail subscribers
• reconcile successful writes with a canonical GET /api/v1/services/{serviceId}
• roll back only the failed mutation revision, preserving newer same-service edits and unrelated concurrent updates
• prevent out-of-order responses from replacing a newer successful local edit
• surface stable typed error codes (SERVICE_WRITE_FAILED, SERVICE_RECONCILE_FAILED) without leaking backend internals
• announce rollback failures through a polite live region
• document the consistency model

Tests

• unit: optimistic -> canonical success
• unit: exact rollback + typed error
• unit: rapid same-key edits with out-of-order responses
• unit: unrelated concurrent service updates
• integration: dashboard optimistic price + isolated rollback
• integration: edit rollback + polite assistive-tech announcement

Validation note

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[woahwhattheheck]

/claim #624

Existing issue-level claim: #624 (comment)

I, @woahwhattheheck, affirmatively claim original-contributor credit for PR #643 and request evaluation for any applicable GrantFox OSS / Official Campaign (FWC26) reward and payment if accepted. Current carrier head: 9fa85b79a0e26886cc05a0af2b58457868533557.