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>

app/Services/FuelPriceService.php

@@ -209,9 +209,9 @@ class FuelPriceService
                 'postcode' => $data['location']['postcode'],
                 'lat' => $data['location']['latitude'],
                 'lng' => $data['location']['longitude'],
-                'amenities' => self::flattenEnabledFlags($data['amenities'] ?? []),
+                'amenities' => $this->flattenEnabledFlags($data['amenities'] ?? []),
                 'opening_times' => $data['opening_times'] ?? null,
-                'fuel_types' => self::flattenEnabledFlags($data['fuel_types'] ?? []),
+                'fuel_types' => $this->flattenEnabledFlags($data['fuel_types'] ?? []),
                 'last_seen_at' => $now,
             ]);
 
@@ -242,7 +242,7 @@ class FuelPriceService
      * @param  array<string, bool>|array<int, string>  $flags
      * @return array<int, string>
      */
-    private static function flattenEnabledFlags(array $flags): array
+    private function flattenEnabledFlags(array $flags): array
     {
         if ($flags === []) {
             return [];