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>

models.py

@@ -125,6 +125,19 @@ class Checkpoint(db.Model):
     def __repr__(self):
         return f'<Checkpoint {self.checkpoint_id}>'
 
+class Preset(db.Model):
+    id = db.Column(db.Integer, primary_key=True)
+    preset_id = db.Column(db.String(100), unique=True, nullable=False)
+    slug = db.Column(db.String(100), unique=True, nullable=False)
+    filename = db.Column(db.String(255), nullable=True)
+    name = db.Column(db.String(100), nullable=False)
+    data = db.Column(db.JSON, nullable=False)
+    image_path = db.Column(db.String(255), nullable=True)
+
+    def __repr__(self):
+        return f'<Preset {self.preset_id}>'
+
+
 class Settings(db.Model):
     id = db.Column(db.Integer, primary_key=True)
     llm_provider = db.Column(db.String(50), default='openrouter') # 'openrouter', 'ollama', 'lmstudio'