docs: add design-rationale commentary to core support classes

Enriches class-level docblocks and inline comments across the shared
utility classes with the "why" behind design decisions that aren't
obvious from reading the code alone:

- Cache       two-tier rationale, atomic-write semantics, failure modes
- Curl        single-use-per-instance rationale, default option choices
- Log         wrapper rationale, redaction expectations for callers
- Database    auto-migration philosophy, schema-versioning approach
- ServerResource  flat-array rationale, interfaces[0]-only limit called
              out for future maintainers, unit-conversion map
- ConfigureService  why a sibling of ModuleFunctions, catalogue caching
              policy, cp-in-constructor reasoning

Pure documentation — no code changes, all files remain lint-clean and
Pint-formatted.

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

modules/servers/VirtFusionDirect/lib/Curl.php

@@ -17,7 +17,20 @@ class Curl
     /** @var array User-supplied cURL options that override defaults */
     private $customOptions = [];
 
-    /** @var array Default cURL options applied to every request */
+    /**
+     * @var array Default cURL options applied to every request.
+     *
+     * Rationale:
+     *   VERIFYPEER/VERIFYHOST: Full TLS chain + hostname validation. Disabling
+     *     either is a common source of MITM bugs, so we never do it silently.
+     *   RETURNTRANSFER: We always want the response body back as a string.
+     *   HEADER off: Callers almost never need headers. Saves a parse cycle.
+     *   NOBODY off: Default to GET-style body-returning requests.
+     *   TIMEOUT 30s: Covers slow API endpoints without letting a hung connection
+     *     block a whole WHMCS request indefinitely.
+     *   CONNECTTIMEOUT 10s: Separate from the total timeout so a failed TCP
+     *     handshake (firewall black-hole) fails fast rather than burning 30s.
+     */
     private $defaultOptions = [
         CURLOPT_SSL_VERIFYPEER => true,
         CURLOPT_SSL_VERIFYHOST => 2,