Make photo ordering atomic so concurrent uploads cannot write the same order value

[rioredwards]

Summary

New photo order is derived from count() + 1, which is race-prone. Concurrent uploads can read the same count and write duplicate order values, producing unstable feed ordering.

Why this matters

• Feed order becomes nondeterministic under concurrent uploads
• Duplicate order values make sort behavior depend on database implementation details
• This gets worse as usage increases

Evidence

• app/db/actions.ts:117-133

Suggested fix

Replace the count-based order assignment with an atomic monotonic strategy. Options include a database sequence, ordering by created_at plus a tiebreaker, or an insert that derives the next order inside a transaction with proper locking.

Acceptance criteria

• Concurrent uploads cannot generate duplicate ordering keys
• Feed ordering is deterministic
• The chosen strategy is documented in code comments or schema notes
• Add a regression test or concurrency-focused integration check if practical