feat(ai): add per-character AI model assignment and identity profiles

Implements deterministic model resolution (column → env map → pool →
default) and adds AI identity fields to characters. Migration backfills
existing rows with deterministic defaults. Controller uses resolved model
and identity block in system prompts for contextual conversations.

config/ai.php

@@ -1,5 +1,33 @@
 <?php
 
+/*
+|--------------------------------------------------------------------------
+| Character model map parser
+|--------------------------------------------------------------------------
+|
+| Turns AI_CHARACTER_MODELS entries such as "Maria=qwen/qwen3.7-flash:free"
+| or "3=deepseek/deepseek-v4-flash-0731" into a lookup array keyed by
+| lowercased character id or name. Invalid pairs are ignored.
+|
+*/
+$parseCharacterModels = static function (string $raw): array {
+    $map = [];
+
+    foreach (explode(',', $raw) as $pair) {
+        $pair = trim($pair);
+        if ($pair === '' || ! str_contains($pair, '=')) {
+            continue;
+        }
+
+        [$key, $model] = array_map('trim', explode('=', $pair, 2));
+        if ($key !== '' && $model !== '') {
+            $map[strtolower($key)] = $model;
+        }
+    }
+
+    return $map;
+};
+
 return [
 
     /*
@@ -50,8 +78,43 @@ return [
         'poolside/laguna-xs-2.1:free',
         'inclusionai/ling-3.0-flash-vl:free',
         'qwen/qwen3.7-flash:free',
+        // Verified working free chat models (live probe of the OpenRouter
+        // catalogue). Safe defaults for per-character assignment.
+        'deepseek/deepseek-v4-flash-0731',
+        'nvidia/nemotron-3.5-lightning:free',
+        'liquid/lfm-2.5-2.6b:free',
+        'openrouter/free',
     ],
 
+    /*
+    |--------------------------------------------------------------------------
+    | Per-Character Model Pool (AI_MODELS)
+    |--------------------------------------------------------------------------
+    |
+    | Comma-separated allow-listed models distributed deterministically
+    | across characters (each character keeps a stable own model). Falls
+    | back to AI_DEFAULT_MODEL when empty.
+    |
+    */
+
+    'models' => array_values(array_filter(array_map(
+        'trim',
+        explode(',', (string) env('AI_MODELS', ''))
+    ))),
+
+    /*
+    |--------------------------------------------------------------------------
+    | Per-Character Model Map (AI_CHARACTER_MODELS)
+    |--------------------------------------------------------------------------
+    |
+    | Explicit "id or name = model" overrides from the environment, e.g.
+    | "Maria=liquid/lfm-2.5-2.6b:free,7=openrouter/free". Takes priority
+    | over the pool; entries outside the allow-list are ignored.
+    |
+    */
+
+    'character_models' => $parseCharacterModels((string) env('AI_CHARACTER_MODELS', '')),
+
     /*
     |--------------------------------------------------------------------------
     | Default Model