chore(config): drop the unused minitems / maxitems database fields

[icemansparks]

Follow-up to the discussion on #33.

Why

AHBConfig::InitializeFromSql() had both reads commented out:

//SetMinItems(WorldDatabase.Query("SELECT minitems FROM mod_auctionhousebot ...
//SetMaxItems(WorldDatabase.Query("SELECT maxitems FROM mod_auctionhousebot ...

So the minitems / maxitems columns were never read. The seller's ceiling comes from AuctionHouseBot.MaxItems in the conf file. Both columns were still being written by .ahbotoptions minitems/maxitems, which made editing them look effective while doing nothing — the exact trap worth removing.

MinItems was worse: GetMinItems() had no caller anywhere. The conf option (AuctionHouseBot.MinItems = 10000), the member, the accessors, the column and the GM command were all completely inert.

Changes

• Removed minItems (member, Get/SetMinItems, conf read, reset, copy-ctor), AuctionHouseBot.MinItems from the conf dist, the minitems GM command, its enum value and its help line.
• Removed the two commented-out SQL reads and the commented minItems debug log.
• .ahbotoptions maxitems keeps working as a runtime-only override — it already recalculates the per-quality maxima via CalculatePercents() — but no longer UPDATEs a column nothing reads. Help text now says "until restart".
• New migration z_2026_08_24_00_drop_minitems_maxitems.sql drops both columns.
• README: corrected the "Market population control" section.

Migration notes

• The migration is guarded via information_schema and re-runnable, and works on MySQL as well as MariaDB (MySQL has no DROP COLUMN IF EXISTS).
• mod_auctionhousebot.sql is deliberately left untouched. That file starts with DROP TABLE IF EXISTS; changing it changes its hash and the DB updater would re-apply it, wiping the tuned prices and percentages in the live table. The consequence is that a fresh install creates the two columns and this migration drops them again immediately afterwards — the z_ prefix keeps it sorted after the base file, same trick z_filter_disabled_and_trash.sql uses.
• Existing mod_ahbot.conf files may still carry a AuctionHouseBot.MinItems line. Harmless — unknown options are ignored — but it can be deleted.

Verification

• Grepped clean: no minItems / minitems references remain in src/, conf/ or docs/.
• Full module compile: the core-build workflow on this PR.
• The migration SQL has not been executed anywhere yet — no MySQL/MariaDB on the box I prepared this on. It should be run against testcore (LXC 113) before it reaches live.

Note on ordering with #33

Independent of #33 (that one only touches GetItemsToSell()); no conflicts expected in either merge order.