config: retire the core.* logging namespace (closes #443)

[lonix]

Closes #443. Picking Path A (delete) as recommended in the issue.

Why delete it

After #440 removed every unread core.* key, the namespace was down to a single survivor: core.cleanup.channel_id. Audit shows it's fully vestigial.

VoiceChannelTruncationService.getNotificationChannel() reads the key, but nothing in the service ever actually sends a notification to that channel — the value flows only into the Database admin-page renderer, which displays "Notification channel: #X" next to a feature that doesn't post anything. Setting or unsetting the key produces zero observable behavior.

Given the v1.0 direction ("admin panel is the source of truth, not a sprawl of Discord channels"), Path A is the right call.

Changes

Schema (src/services/config-schema.ts)

• Drop core.cleanup.channel_id from ConfigSchema, defaultConfig, settingsMetadata.
• Drop the core entry from categoryMetadata (no keys left in it).
• cleanupUnknownSettings() will purge any orphan DB rows on next start. The Mongoose Config model keeps "core" in its category enum so the validator accepts those rows during the transition window — same pattern as the retired amikool / fun enum slots.

Service (src/services/voice-channel-truncation.ts)

• Remove getNotificationChannel(). Its single caller used the result only for display, with no real downstream effect.

Database admin page (src/web/admin-views.ts, read-only-routes.ts)

• DatabaseProps.trunk loses the notificationChannel field; the page no longer renders the "Notification channel" row.
• Other dbtrunk diagnostics (schedule, last run, retention, history) unchanged.
• Route handler drops the corresponding truncation.getNotificationChannel() await + the now-unused fetchChannelData call.

Docs (SETTINGS.md)

• Drop the entire "📝 Discord Logging" section, which still documented the four core.* sub-namespaces Config: delete dead/unused config keys #440 had already removed (longstanding doc drift cleaned up here).
• Drop matching TOC entry and quick-reference block.

Tests

• Drop the two core.cleanup.channel_id assertions in config-schema.test.ts.
• Drop notificationChannel: null from the two renderDatabasePage fixtures in admin-views.test.ts.

Verification

• npm test — 743 passed, 1 skipped, 0 failed.
• npx tsc --noEmit — clean.
• npm run lint — 0 errors.
• npm run format:check — clean.
• grep -rn 'core\\.' src returns nothing.
• Manual on a deployment that had core.cleanup.channel_id set: on next start expect cleanupUnknownSettings() log 🧹 Found 1 unknown/old settings... listing it.
• Manual: /admin/database no longer shows the "Notification channel" row, other rows unchanged.

Test plan

• Existing tests still pass; behavior unchanged for everything outside the vestigial code path.
• Manual UI check on /admin/database.

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