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/example_01.json

@@ -0,0 +1,84 @@
+{
+  "preset_id": "example_01",
+  "preset_name": "Example Preset",
+  "character": {
+    "character_id": "aerith_gainsborough",
+    "use_lora": true,
+    "fields": {
+      "identity": {
+        "base_specs": true,
+        "hair": true,
+        "eyes": true,
+        "hands": true,
+        "arms": false,
+        "torso": true,
+        "pelvis": false,
+        "legs": false,
+        "feet": false,
+        "extra": "random"
+      },
+      "defaults": {
+        "expression": "random",
+        "pose": false,
+        "scene": false
+      },
+      "wardrobe": {
+        "outfit": "default",
+        "fields": {
+          "full_body": true,
+          "headwear": "random",
+          "top": true,
+          "bottom": true,
+          "legwear": true,
+          "footwear": true,
+          "hands": false,
+          "gloves": false,
+          "accessories": "random"
+        }
+      }
+    }
+  },
+  "outfit": {
+    "outfit_id": null,
+    "use_lora": true
+  },
+  "action": {
+    "action_id": "random",
+    "use_lora": true,
+    "fields": {
+      "full_body": true,
+      "additional": true,
+      "head": true,
+      "eyes": false,
+      "arms": true,
+      "hands": true
+    }
+  },
+  "style": {
+    "style_id": null,
+    "use_lora": true
+  },
+  "scene": {
+    "scene_id": "random",
+    "use_lora": true,
+    "fields": {
+      "background": true,
+      "foreground": "random",
+      "furniture": "random",
+      "colors": false,
+      "lighting": true,
+      "theme": false
+    }
+  },
+  "detailer": {
+    "detailer_id": null,
+    "use_lora": true
+  },
+  "look": {
+    "look_id": null
+  },
+  "checkpoint": {
+    "checkpoint_path": null
+  },
+  "tags": []
+}

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
+        }
+    }
+}