chore: audit nits — PlanFeatures, test boot, EIA log, static method

Audit items #15, #16, #20, #22.

#15 — AuthController::me and UserResource form/table now read tier via
PlanFeatures::for($user)->tier() instead of Plan::resolveForUser($user)
->name. Tiers.md: PlanFeatures is the single entitlement gate.

#16 — Moved SQLite GREATEST/LEAST PHP-backed function registration from
AppServiceProvider::boot to tests/TestCase::setUp. Production app boot no
longer checks the DB driver name.

#20 — FetchOilPrices: added Log::warning on EIA fallback and Log::error
on both-providers-failed so primary-source reliability can be trended
beyond the cron output buffer.

#22 — FuelPriceService::flattenEnabledFlags is now an instance method,
matching the rest of the class. No external callers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/TestCase.php

@@ -3,10 +3,26 @@
 namespace Tests;
 
 use Illuminate\Foundation\Testing\TestCase as BaseTestCase;
+use Illuminate\Support\Facades\DB;
 use Laravel\Fortify\Features;
 
 abstract class TestCase extends BaseTestCase
 {
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        // SQLite lacks GREATEST/LEAST scalar functions — register PHP-backed
+        // shims so the haversine and other math expressions used in
+        // production-style queries run identically in :memory: tests.
+        // Idempotent: registering twice on the same PDO is harmless.
+        if (DB::connection()->getDriverName() === 'sqlite') {
+            $pdo = DB::connection()->getPdo();
+            $pdo->sqliteCreateFunction('GREATEST', fn (...$args) => max($args), -1);
+            $pdo->sqliteCreateFunction('LEAST', fn (...$args) => min($args), -1);
+        }
+    }
+
     protected function skipUnlessFortifyHas(string $feature, ?string $message = null): void
     {
         if (! Features::enabled($feature)) {