fix(graphql): default MAX_PRODUCTS_PER_REQUEST to 5000

[andreacappelletti97]

#60 capped each request at 1,000 products. c3x pricing sync enumerates the catalog with limit:5000 pages, and released CLIs treat a page shorter than 5,000 as the last one. So since that deploy, every sync kept only the first 1,000 products of each service and region, and c3x estimate --offline priced most resources at the $0 stub. I reproduced it: an m5.xlarge came out at $0 offline versus $140.16 live, and a synced us-east-1 cache had 4,455 prices instead of 22,364.

• The default is now 5,000, one enumeration page, so released CLIs sync fully again. Ordinary lookups are unaffected; they use limit:50.
• The CLI is also fixed so any cap size works (fix(pricing): page through the whole catalog in pricing sync c3x#104). This change is what repairs the versions already installed.
• Updated docker-compose.yml, .env.example, README and CHANGELOG. The ops: query cost limits, cheaper scrapes, GIN-served lookups, monitoring, backups #60 entry wrongly said the CLI was unaffected; it now says pricing sync pages 5,000 at a time.

Deploy note: the VPS .env has no MAX_PRODUCTS_PER_REQUEST line, so rebuilding picks up the new default. Alternatively, add MAX_PRODUCTS_PER_REQUEST=5000 to .env and restart the API.