Config: demote magic-constant config keys to module-local constants

[lonix]

Problem

Several config keys exist because someone needed a number and reached for getNumber(), not because operators ever change them in production. They add noise to the Settings page and demand label/type metadata for #436/#439 that won't pay off.

Confirmed candidates:

• quotes.cleanup_interval (default 5 min) — internal cleanup loop tick in src/services/quote-channel-manager.ts.
• notices.cleanup_interval (default 5 min) — same pattern in src/services/notices-channel-manager.ts:242.

Likely candidates (confirm during implementation):

• quotes.max_length (default 1000) — Discord's hard limit is 2000; this is an internal bound, not operator preference.
• voicechannels.ownership.grace_period_seconds (default 30s) — internal transfer-delay constant; arguable but very unlikely to be tuned in practice.

Proposed change

• Move the confirmed candidates from ConfigSchema to consts in their respective modules.
• For the likely candidates: decide per-key during implementation. Default plan: demote both.
• Add a startup migration step to drop orphan rows from the configs collection.
• Remove from defaultConfig and settingsMetadata.

Acceptance

• Demoted keys no longer in ConfigSchema.
• Modules use module-local constants; values unchanged.
• Existing tests still pass; if any test asserts these values, update to import the new constant.

Notes

• Same blast-radius pattern as Config: delete dead/unused config keys #440 (dead-keys cleanup) — shrinking the operator surface area.