v0.2.0:3 — durable payment-provider switching (Option B)

Closes the gap from :2 where Connect Zaprite swapped the
in-memory provider but BTCPay would silently re-take active on
the next daemon restart (because the boot-time loader picked
BTCPay first whenever btcpay_config was present, regardless of
operator intent).

What changed:

**New settings key `active_payment_provider`** in the existing
settings table. Records the operator's last explicit choice
('btcpay' | 'zaprite' | NULL = no preference). Both
btcpay_config and zaprite_config can coexist; the flag is what
determines which one the daemon loads.

**Boot-time loader respects the preference.** main.rs now reads
the flag at startup. If set to 'zaprite', Zaprite wins; if set to
'btcpay', BTCPay wins; if unset (legacy installs), falls back to
the previous BTCPay-first ordering. Cross-load fallbacks log a
WARN and try the other provider — operators with a stale flag
pointing at a wiped config don't boot unconfigured.

**Connect endpoints write the preference.**
- finish_connect (BTCPay) now sets the flag to 'btcpay' on
  successful authorize-callback completion.
- ZapriteAuthorize::connect now sets the flag to 'zaprite' on
  successful API-key validation.
- Both Disconnect endpoints clear the flag IF it pointed at the
  provider being disconnected — but leave it alone if it pointed
  at the OTHER provider (different operator intent).

**New endpoints for fast switching without re-Connect:**
- GET /v1/admin/payment-provider/status — both configs' state +
  current preference + runtime active provider, in one call.
- POST /v1/admin/payment-provider/activate { provider: "btcpay" |
  "zaprite" } — flips the active provider and the flag together,
  without going through the full Connect flow. 400 if the named
  provider isn't configured (operator must run Connect first).

**New StartOS Actions** under existing groups:
- "Activate BTCPay" (in BTCPay group)
- "Activate Zaprite" (in Zaprite group)
Both call the new activate endpoint. Operators with both
providers configured can flip back and forth in one click.

**Test:** payment_provider_preference_round_trip pre-seeds both
configs, walks through Activate-Zaprite → Activate-BTCPay →
attempt-Activate-on-wiped-config → bad-provider-name → manual
write/read of the preference key. Pins the contract.

Test count: 42 (was 41; +1).

Migration not needed — settings table from 0005 already has the
key/value/updated_at shape we need.

startos/actions/index.ts

@@ -20,6 +20,7 @@
 
 import { sdk } from '../sdk'
 import { activateLicense, showLicenseStatus } from './activateLicense'
+import { activateBtcpay, activateZaprite } from './activatePaymentProvider'
 import { btcpayStatus, configureBtcpay, disconnectBtcpay } from './configureBtcpay'
 import { configureZaprite, disconnectZaprite, zapriteStatus } from './configureZaprite'
 import { setOperatorName } from './setOperatorName'
@@ -33,10 +34,12 @@ export const actions = sdk.Actions.of()
   // BTCPay setup (Bitcoin-only payments via your own BTCPay Server)
   .addAction(configureBtcpay)
   .addAction(btcpayStatus)
+  .addAction(activateBtcpay)
   .addAction(disconnectBtcpay)
   // Zaprite setup (Bitcoin + fiat-card payments via Zaprite's broker)
   .addAction(configureZaprite)
   .addAction(zapriteStatus)
+  .addAction(activateZaprite)
   .addAction(disconnectZaprite)
   // Keysat self-license (Keysat-licenses-Keysat)
   .addAction(activateLicense)