docs: fix ENABLE_LOGGING variable name in CONFIGURATION.md

[mshddev]

The Monitoring section of docs/CONFIGURATION.md documents MYSQL_ENABLE_LOGGING, but the code reads ENABLE_LOGGING:

// src/utils/index.ts:5
const ENABLE_LOGGING =
  process.env.ENABLE_LOGGING === "true" || process.env.ENABLE_LOGGING === "1";

Setting the documented name has no effect — logging silently stays off. Both .env.dist and RELEASE.md already use ENABLE_LOGGING, so this docs page was the only place carrying the wrong name.

Changes

• MYSQL_ENABLE_LOGGING → ENABLE_LOGGING, in both the Monitoring table and the Advanced Config Example
• Removed MYSQL_LOG_LEVEL and MYSQL_METRICS_ENABLED — neither appears anywhere in the source, so they are documented but unimplemented
• Corrected the description. The flag gates diagnostic logging (startup, pool creation, connection acquire/release, errors), not query logging — the SQL text is never logged, apart from failed queries where the mysql2 error object carries it
• Noted the accepted values, since ENABLE_LOGGING=yes would silently do nothing

Verification

$ grep -rn "MYSQL_ENABLE_LOGGING" --include="*.ts" --include="*.js" . | grep -v node_modules
(no results)

Same for MYSQL_LOG_LEVEL and MYSQL_METRICS_ENABLED.

Not addressed here

MYSQL_POOL_SIZE, MYSQL_QUERY_TIMEOUT, MYSQL_CACHE_TTL and MYSQL_RATE_LIMIT also appear in the Advanced Config Example but have no source references either. Left alone to keep this diff focused — happy to follow up if you want them cleaned out too. (Pool size in particular is hardcoded to 10 at src/config/index.ts:224.)

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