fix(db): don't hint missing ban indexes in PruneBans (#1578)

[rumblefrog]

Fixes #1578.

Problem

#1577 (2.2.1) added FORCE INDEX (type_authid) / FORCE INDEX (type_ip) to the submission lookup in PruneBans(). Some long-upgraded installs don't have those two indexes on :prefix_bans. 702.php creates them, but the updater only runs scripts above the stored config.version, so an install that moved past 702 without running it never got them. MariaDB treats a hint on a missing index as error 1176, not a no-op. As a result, the Ban List page and the add / edit ban flows (PruneBans() callers: page.banlist.php, admin.edit.ban.php, api_bans_add) fatal with:

SQLSTATE[42000]: 1176 Key 'type_authid' doesn't exist in table 'BSteam'

Fix

• PruneBans(): drop the index hints. When the indexes exist, the optimizer picks them for each arm on its own. The reviewer checked this against a 200k-ban table after ANALYZE.
• web/updater/data/811.php (registered in store.json): checks information_schema.STATISTICS and adds whichever of type_authid / type_ip is missing, in a single ALTER TABLE. This makes upgraded installs match struc.sql. It's idempotent and portable (no MariaDB-only ADD INDEX IF NOT EXISTS), and it's a no-op on fresh installs, which already have both indexes.
• Docs: a new "Key doesn't exist" entry in troubleshooting/database-errors.md with the manual SQL workaround, plus an AGENTS.md Database-conventions rule against index hints on indexes that upgraded installs may lack.

Tests

web/tests/integration/BansCompositeIndexesTest.php:

• PruneBans() archives matching Steam / IP submissions with both indexes dropped. On main this test fails with the exact 1176 error from the issue.
• The migration recreates both indexes with the struc.sql column order.
• If only one index is missing, the migration adds just that one.
• Re-running the migration adds no duplicates.
• tearDown restores the schema.

Local gates: PHPStan is clean. PHPUnit passes apart from the six PluginVersionResolveTest cases, which fail only because the dev container doesn't mount game/; they're unrelated and pass in CI. UpdaterMigrationPortableSqlTest passes.

An adversarial review pass found no blockers. Its findings are addressed in the second commit: the corrected list of affected surfaces, a single combined ALTER so old MyISAM tables are rebuilt once, the type_ip error variant in the docs, and a tighter rationale for the existing :prefix_comms FORCE INDEX (created) hints.