Add Preset Library feature

Presets are saved generation recipes that combine all resource types
(character, outfit, action, style, scene, detailer, look, checkpoint)
with per-field on/off/random toggles. At generation time, entities
marked "random" are picked from the DB and fields marked "random" are
randomly included or excluded.

- Preset model + sync_presets() following existing category pattern
- _resolve_preset_entity() / _resolve_preset_fields() helpers
- Full route set: index, detail, generate, edit, upload, clone, save_json, create (LLM), rescan
- 4 templates: index (gallery), detail (summary + generate), edit (3-way toggle UI), create (LLM form)
- example_01.json reference preset + preset_system.txt LLM prompt
- Presets nav link in layout.html

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

data/presets/preset.json

@@ -0,0 +1,29 @@
+{
+    "preset_id": "example_01",
+    "preset_name": "Example Preset",
+    "prompt":{
+        "character": {
+            "character_id": "aerith_gainsborough",
+            "identity": {
+                "base_specs": true,
+                "hair": true,
+                "eyes": true
+                ...
+            },
+            "defaults": {
+                "expression": false,
+                "pose": false,
+                "scene": false
+            },
+            "wardrobe": {
+                "outfit_id": "default",
+                "outfit": {
+                    "headwear": true, 
+                    "accessories": true
+                    ...
+                }
+            },
+            "use_lora": true
+        }
+    }
+}