feat(addons): generic per-addon settings store (AddonSetting + addon_settings + settings UI)

[hedon77]

What

Extracts the reusable addon-framework settings layer so addons can store configuration (API keys, endpoints, toggles) without depending on a specific addon.

• \AddonSetting\ model (values encrypted at rest via \EncryptedValue\ cast).
• \�ddon_settings\ migration (one row per addon/setting).
• \AddonManager::settings()\ / \saveSettings().
• \AddonController::saveSettings()\ + settings passed to the addon page.
• Settings form on the addon page (text, textarea, password, checkbox, select).
• \�ddons.modules.settings\ route.
• Renames the admin section from \Addon Modules\ to \Extensions.

Why

This is the shared base for the KSeF (#20) and Company Lookup (#19) addons. It lets each addon live in its own PR without duplicating the settings plumbing.

Notes

• Depends on nothing; merge before feat(company-lookup): add NIP company lookup (GUS, MF, CEIDG, OpenBRIS) #19 and feat(ksef): add KSeF 2.0 e-invoicing integration #20.

[hedon77]

Review — ready to merge

Generic addon settings store (AddonSetting + addon_settings migration + settings UI). Clean, framework-only.

Sugestia: zmerguj jako pierwszy — jest bazą dla #19 (company-lookup) i #20 (KSeF).