feat(license): replace local auto-trial with Lemon Squeezy hosted trial flow (#755)

Fresh installs land on the Community tier. The 14-day Admiral trial is now
issued by Lemon Squeezy via their hosted checkout: the user enters email +
card, receives a license key by email, and pastes it into the existing
Settings > License activation field.

Backend changes:
- LicenseService.initialize() no longer auto-creates a license_status='trial'
  row on first boot. It now only ensures an instance_id exists and starts
  periodic validation.
- Drop the TRIAL_DURATION_DAYS constant.
- Drop the status='trial' early-return in getVariant() so LS-issued trials
  resolve through the normal variant metadata path (variant_name / product_name).
- Trial branches in getTier() and getLicenseInfo() are retained for future
  work that may detect trial state from Lemon Squeezy metadata; they are
  currently unreachable via the Sencho code paths.

Frontend changes:
- Settings > License surfaces a new "Try Admiral free for 14 days" CTA block
  with Start monthly trial and Start annual trial buttons that open Lemon
  Squeezy hosted checkout. The CTA is visible only when the user has no paid
  access and is not already on a trial.
- Reserve the Admiral upgrade card for the Skipper-active upgrade path so
  unlicensed users see one Admiral path (the trial CTA) instead of two.
- Pull the inline Lemon Squeezy checkout URLs into named module constants so
  the Skipper, Admiral monthly, and Admiral annual endpoints are defined in
  one place.

Test changes:
- license-service.test.ts covers the no-auto-trial startup path and updates
  the trial-variant test to reflect the metadata-driven resolution.
- afterAll in the initialize() describe block calls destroy() so the 72-hour
  validation interval does not leak into sibling test files.

Docs:
- Rewrite the Free trial section in features/licensing.mdx to document the
  new LS checkout flow (email + card required, auto-converts on day 14 unless
  cancelled).
- Add an operations/troubleshooting entry for cases where the trial license
  key email does not arrive.

docs/operations/troubleshooting.mdx

@@ -166,6 +166,19 @@ To re-test connectivity after making changes, open **Profile > Settings > Nodes*
 
 ---
 
+## I didn't receive my trial license key after checkout
+
+**Symptom:** You completed the Lemon Squeezy trial checkout but no license key email arrived.
+
+**Checks in order:**
+
+1. **Check your spam / junk folder.** Lemon Squeezy emails come from `no-reply@lemonsqueezy.com`; some providers flag them aggressively.
+2. **Verify the email address you used at checkout.** If you made a typo, the key was sent to the wrong address. Contact `licensing@sencho.io` with your order ID (from the checkout receipt) and we can resend or issue a new key.
+3. **Wait a few minutes.** Delivery is usually within 60 seconds but can take up to 10 minutes during peak load.
+4. **Still missing after 15 minutes?** Email `licensing@sencho.io` with the transaction ID from your card statement and we will resend the key.
+
+---
+
 ## Paid features are still locked after activation
 
 **Symptom:** Features remain locked even though you activated a valid license key.