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>

templates/layout.html

@@ -23,6 +23,7 @@
                 <a href="/scenes" class="btn btn-sm btn-outline-light">Scenes</a>
                 <a href="/detailers" class="btn btn-sm btn-outline-light">Detailers</a>
                 <a href="/checkpoints" class="btn btn-sm btn-outline-light">Checkpoints</a>
+                <a href="/presets" class="btn btn-sm btn-outline-light">Presets</a>
                 <div class="vr mx-1 d-none d-lg-block"></div>
                 <a href="/create" class="btn btn-sm btn-outline-success">+ Character</a>
                 <a href="/generator" class="btn btn-sm btn-outline-light">Generator</a>