Add Checkpoints Gallery with per-checkpoint generation settings

- New Checkpoint model (slug, name, checkpoint_path, data JSON, image_path)
- sync_checkpoints() loads metadata from data/checkpoints/*.json and falls
  back to template defaults for models without a JSON file
- _apply_checkpoint_settings() applies per-checkpoint steps, CFG, sampler,
  base positive/negative prompts, and VAE (with dynamic VAELoader node
  injection for non-integrated VAEs) to the ComfyUI workflow
- Bulk Create from Checkpoints: scans Illustrious/Noob model directories,
  reads matching HTML files, uses LLM to populate metadata, falls back to
  template defaults when no HTML is present
- Gallery index with batch cover generation and WebSocket progress bar
- Detail page showing Generation Settings and Base Prompts cards
- Checkpoints nav link added to layout
- New data/prompts/checkpoint_system.txt LLM system prompt
- Updated README with all current galleries and file structure
- Also includes accumulated action/scene JSON updates, new actions, and
  other template/generator improvements from prior sessions

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

data/scenes/mysterious_4lch3my_sh0p_i.json

@@ -1,38 +1,35 @@
 {
   "scene_id": "mysterious_4lch3my_sh0p_i",
   "scene_name": "Mysterious 4Lch3My Sh0P I",
-  "description": "A dimly lit, cluttered interior of an apothecary filled with bubbling potions, ancient tomes, and strange artifacts.",
+  "description": "A dimly lit, mysterious alchemy shop filled with ancient books, glowing potions, and strange magical artifacts like preserved eyes and enchanted daggers.",
   "scene": {
-    "background": "indoors, room, wooden walls, stone floor, filled shelves, cobwebs, hanging plants, drying herbs",
-    "foreground": "counter, glass bottle, potion, glowing liquid, open book, scroll, skull, crystals, mortar and pestle",
+    "background": "indoors, alchemy laboratory, cluttered, bookshelf, potion, glass bottle, spider web, stone wall, mist, shelves, ancient",
+    "foreground": "wooden counter, open book, grimoire, mortar and pestle, crystal ball, dagger, jar, cauldron, magic circle, glowing runes, steam",
     "furniture": [
+      "wooden shelves",
       "wooden counter",
-      "bookshelf",
-      "display cabinet",
-      "stool"
+      "stone walls"
     ],
     "colors": [
-      "dark brown",
-      "emerald green",
-      "amethyst purple",
-      "gold"
+      "deep purple",
+      "shimmering gold",
+      "eerie green",
+      "brown",
+      "dark grey"
     ],
-    "lighting": "dim, candlelight, lantern, glowing, cinematic lighting, volumetric lighting, shadows",
-    "theme": "fantasy, magic, alchemy, mystery, steampunk"
+    "lighting": "dimly lit, candlelight, flickering, glowing, volumetric lighting",
+    "theme": "fantasy, mystery, arcane, magic, ancient"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/mysterious_4lch3my_sh0p-i.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "mysterious_4lch3my_sh0p-i"
+    "lora_triggers": "mysterious 4lch3my sh0p"
   },
   "tags": [
-    "alchemy",
-    "shop",
-    "magic",
-    "fantasy",
+    "mysterious alchemy shop",
     "interior",
-    "detailed",
-    "cluttered",
-    "mysterious"
+    "fantasy",
+    "background",
+    "magic shop"
   ]
 }