feat(perf): If-Modified-Since → 304 handling for Last-Modified routes

[julianken]

Follow-up to #418. Now that Last-Modified is emitted, implement the conditional-request handling: when proxy.ts receives a request with If-Modified-Since header, compare to Payload doc's updatedAt and return HTTP 304 (no body) when not modified.

Implementation requires an extra cheap Payload lookup in proxy.ts (the same field-projected updatedAt query the #418 proxy already does for the response-header path, but called before NextResponse.next() to gate the request). For listing routes the comparison is against the ISR-snapshot floor captured in proxy.ts.

Why this is non-trivial:

• Per Why am I getting 'RSC' code instead of content after implementing custom headers in my middleware? vercel/next.js#82790, Next does not auto-convert presence of Last-Modified into 304 short-circuits.
• The gated lookup duplicates the Payload query (mitigated by Payload's existing local-memo cache during the request).
• Listing routes need a separate 304 strategy since they use the ISR-snapshot floor, not a per-doc updatedAt.
• The 304 response must mirror the headers a 200 would carry (Last-Modified, ETag, Cache-Control) per RFC 7232.

Acceptance criteria:

• proxy.ts compares If-Modified-Since to the same updatedAt source used for the 200 path
• Returns HTTP 304 (no body, full headers) when the doc has not been modified
• Tests: unit (mock both header presence and updatedAt) + E2E (curl with -H 'If-Modified-Since: ')

Closes the bandwidth-savings half of the original Last-Modified work.