feat(admin): issue #144 — seed phrases for regular admin, commission-gated + Mercuryo payment
- overview: commissionWallets (env) + mercuryoUrl in response
- seeds API: getAuth (regular admin), locked=true + no mnemonics when commissionDue>0, full when 0
- export-seeds: 403 when commissionDue>0
- UI: seed tab for all admins; payment block (past payments, difference, commission wallet selector, Mercuryo card button, I've paid -> record-payment); full table + CSV when unlocked
- super_admin: full access regardless (informational warning)
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