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.

licensing-service/src/api/mod.rs

@@ -71,6 +71,7 @@ pub mod tier;
 pub mod validate;
 pub mod community;
 pub mod db_info;
+pub mod payment_provider;
 pub mod rates_admin;
 pub mod recover;
 pub mod zaprite_authorize;
@@ -245,6 +246,17 @@ pub fn router(state: AppState) -> Router {
             "/v1/admin/zaprite/status",
             get(zaprite_authorize::status),
         )
+        // Provider-agnostic active-payment-provider control.
+        // Operators with both BTCPay and Zaprite configured can flip
+        // the active one without re-running Connect.
+        .route(
+            "/v1/admin/payment-provider/status",
+            get(payment_provider::status),
+        )
+        .route(
+            "/v1/admin/payment-provider/activate",
+            post(payment_provider::activate),
+        )
         // Zaprite webhook landing — operator points Zaprite's
         // webhook setting at this URL. Same handler as
         // /v1/btcpay/webhook because the underlying validate_webhook