refactor(storage): make database creation cancellable

[pablontiv]

Related to #48 — final storage phase.

Summary

• adds OpenContext while preserving Open as the context.Background() wrapper
• initializes new V1–V16 databases atomically in one transaction with cancellation-safe rollback and cleanup
• linearizes publication at the canonical hard link so concurrent openers adopt exactly one complete database
• propagates context through private creation, connection checks, publication, and recovery destination construction
• preserves existing-database migration behavior and wrapper compatibility

Validation

• just ci — pass, 85.3% total coverage
• go test -race -count=1 ./internal/storage — pass
• focused deterministic cancellation, rollback, publication, concurrency, permissions, URI, and recovery tests — pass
• review candidate accepted after the publication ownership race fix and equivalent rebase onto current main