feat: per-wallet mail text overrides for RealUnit

[TaprootFreak]

Summary

Adds per-wallet override mechanism for user-mail texts so partner wallets (RealUnit being the first) can ship their own subjects, salutations and body texts without forking the trigger services or introducing new MailType values.

What changed

• MailFactory.translate(...) is now wallet-aware. When a walletName is passed it tries mail.wallets.<walletname>.<key> first and falls back to the default key. getMailAffix, mapMailAffix and translateParams propagate the walletName, and createUserV2Mail / createPersonalMail pull it from wallet.name automatically.
• mail.json (DE + EN) gets a new wallets.realunit block with overrides for general (closing / support / welcome / thanks), purchase / sale / refund / processing flows plus KYC failure, missing-data, reminder, and email-verification reminder. All texts use formal Sie and the RealUnit Schweiz AG closing.
• realUnit.hbs mail layout (copy of onChainLabs.hbs with RealUnit logo URL swapped). Layout/colors are unchanged for now — RealUnit branding can be tuned in a follow-up once they hand over the assets.
• config.ts flips mail.wallet.RealUnit.template from 'user-v2' to 'realUnit'.

What this maps to (RealUnit template doc v1.1)

Doc #	Trigger / DFX-Event	Override key
#1 Kauf abgeschlossen	BUY_CRYPTO_COMPLETED	payment.crypto_output.*
#2 Verkauf gestartet	BUY_FIAT_PROCESSING	payment.processing.*
#3 Monatslimit	BUY_FIAT_PENDING	payment.chargeback.reasons.monthly_limit
#4 Jahreslimit	BUY_FIAT_PENDING	payment.chargeback.reasons.annual_limit
#5 Einzahlung in Prüfung	BUY_CRYPTO_PENDING	payment.fiat_input.*
#7 Video-KYC	KYC_FAILED	kyc.failed.*
#8 KYC-Daten fehlen	KYC_MISSING_DATA	kyc.missing_data.*
#9 Mail-Bestätigung Reminder	EMAIL_VERIFICATION_REMINDER	verification_code.email.*
#11 KYC-Reminder	KYC_REMINDER	kyc.reminder.*
#12 Rückerstattung Limite	BUY_FIAT_RETURN	payment.chargeback.fiat.*
#15 Verkauf nicht möglich	BUY_FIAT_RETURN	payment.chargeback.crypto.*
#17 Verkaufserlös ausgezahlt	BUY_FIAT_COMPLETED	payment.fiat_output.*

#6 (Zahlungsproblem) and #13 (Aktion ausstehend) are intentionally NOT overridden — they should be disabled per-wallet via wallet.disabledMailTypes once this lands.

Open points (waiting on RealUnit / DAS)

• Final support URL — currently assumed https://support.realunit.ch.
• App path for KYC/flow-of-funds entry — currently text says "Menü → Profil → Verifizierung".
• Consolidation of Hotfix #8 / Feature/log columns #10 / Feature/log columns #11 (kept as three separate texts for now).
• Slots Feature/kyc mail #14 and Feature/ref percent #16 in the v1.0 doc (numbering gap, no template provided).
• RealUnit branding assets for the HBS layout (logo URL is a placeholder, layout colours still match onChainLabs).
• Whether EN should ship in phase 1 alongside DE (added preemptively).

Test plan

• DEV deploy: trigger a KYC, CFP, removed lazy, bugfixes, refactoring #1-style buy completion mail for a RealUnit user and verify subject, salutation and closing render with RealUnit overrides.
• Trigger a non-RealUnit user mail and verify default DFX texts are unchanged.
• Trigger a Feature/log columns #11 KYC-reminder for a RealUnit user and verify both kyc.reminder.title and general.support come from the override block.
• Inspect rendered HTML in Outlook/Gmail/iOS Mail (RealUnit logo loads, layout intact).
• EN-language RealUnit user receives the EN override texts.

Out of scope

• DB schema changes — none needed; everything is config-/i18n-driven.
• Refactor of MailFactory beyond the override hook.
• New MailType or MailContext values.
• FR/IT translations (phase 2).

[TaprootFreak]

Doppelarbeit — wird ersetzt durch den älteren Draft-PR #3598 (feat/realunit-mail-template), der bereits weiter gediehen ist (separate mail-realunit.json, body-text-injection in der Factory, Preview-Script).