Parsing and validation of the Prefer request header for the read path (the write path's Prefer handling lives in Bier.Mutation).

Mirrors PostgREST v16.0's ApiRequest.Preferences semantics:

• a preference is recognized by its WHOLE token, not by its key: acceptedPrefs is the list of canonical <key>=<value> strings the ToHeaderValue instances produce, and isUnacceptable tests membership in it (Preferences.hs#L145-L163). Only timezone= and max-affected=, whose values are free-form, are matched by prefix. A token with a known key but an unknown value (count=none, return=bogus) is therefore invalid, not merely ignored;
• handling=strict rejects the whole request (400 PGRST122) when ANY supplied preference is invalid. The error details lists the offending tokens verbatim, comma-separated;
• handling=lenient (or no handling) silently ignores invalid preferences;
• timezone=<value> is never an invalid preference. v16.0 dropped the pg_timezone_names membership test that v14.12 applied (fromHeaders lost its TimezoneNames argument along with isTimezonePrefAccepted, Preferences.hs#L129-L163), so every value is accepted as a preference and handed to PostgreSQL as the session TimeZone. A value PostgreSQL rejects therefore surfaces as an ordinary database error (SQLSTATE 22023 -> 400), not as a PGRST122 — and handling has nothing left to suppress, which is why the docs say "handling=lenient is ignored for timezone. Invalid time zones always return an error";
• the applied preferences are echoed in Preference-Applied in PostgREST's canonical prefsVals order — resolution, missing, representation, count, transaction, handling, timezone, max-affected (Preferences.hs#L179-L188) — never in request order. On a read only count, handling and timezone can apply, so the echo is count before handling before timezone.

Numeric UTC offsets (+05:30, -4) are consequently valid too: they are not members of pg_timezone_names but PostgreSQL accepts them as a TimeZone, and the echo carries the raw preference token rather than a normalized zone name.