fix(contacts): preserve preferences during pending opt-in

[vladbisceanu]

Problem

The documented double-opt-in flow currently needs two secret-key calls. A public one-call track payload with subscribed: false would unsubscribe an existing active contact, while omitting it creates a new contact subscribed.

Solution

• add preserveExistingSubscription to the public track schema
• apply subscribed only on first creation when that option is true
• preserve the elected contact state if another request wins the concurrent create
• update the contact concepts, recipe, and OpenAPI contract

The race path intentionally returns the elected row without replaying the losing request metadata. This matches #469: stale concurrent payloads must not overwrite winner-owned contact data. The fork version composes this behavior with that broader race hardening.

Proof

• ContactService suite: 62/62 passed
• yarn workspace api build: passed
• yarn lint: passed (existing warnings only)
• final-diff review completed; its metadata-replay suggestion was dismissed for the winner-ownership reason above

Fork tracking: vladbisceanu#16
Fork implementation: vladbisceanu#17
Related concurrency hardening: #469