Add AI settings API with key management

GET/PATCH /api/v1/settings/ai return and change provider, base_url,
model, temperature and system_prompt, with read-only api_key_last4 and
key_verified_at. PATCH requires a preset provider, temperature 0-1 in
0.1 steps and a system_prompt of at most 8000 characters.
PUT /settings/ai/api-key seals the key with secretbox and records its
last four characters; POST /settings/ai/verify calls Provider.Verify and
records key_verified_at or returns the provider's typed error;
GET /settings/ai/models lists the live models; POST
/settings/ai/reset-prompt restores household.DefaultPersonaPrompt.

The new internal/ai/aisettings service also builds the harness.Config
for each skill run from the saved settings. presets.ForAccount builds a
provider's ListModels and Verify without a model, so a key can be
checked and a model picked before one is chosen.

Tests scan every response body from these routes for the full key, and
a harness run with the Fake provider uses the PATCHed model and
temperature.

Maestro task: 176b2a30-6e90-4339-b430-5b8967b2fd32

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/household/defaults.go

@@ -11,7 +11,8 @@ import (
 )
 
 // DefaultPersonaPrompt is the default AI system prompt (CTXD PROMPTS.MD §4).
-// SeedAISettings stores it on first boot and the Admin reset restores it.
+// SeedAISettings stores it on first boot and aisettings.Service.ResetPrompt
+// restores it.
 const DefaultPersonaPrompt = `You are MealQuest, a dinner planning assistant for one household. Suggest dinners
 similar to their regular rotation, nudging them to try something a little new each
 week. Prefer weeknight-friendly recipes with common grocery-store ingredients.