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/arcade___illustrious.json

@@ -1,34 +1,37 @@
 {
   "scene_id": "arcade___illustrious",
   "scene_name": "Arcade Illustrious",
-  "description": "A vibrant, bustling interior of a Japanese-style game center illuminated by the glow of retro arcade cabinets and neon signs.",
+  "description": "A vibrant, dimly lit arcade filled with rows of classic game cabinets and neon lights.",
   "scene": {
-    "background": "arcade, indoors, game center, crowded, scenery, posters, ceiling lights, patterned carpet, rows of machines",
-    "foreground": "arcade cabinet, claw machine, joystick, buttons, screen, neon signage, glowing",
+    "background": "indoors, checkered_floor, neon_lights, dark_background, poster",
+    "foreground": "arcade_cabinet, joystick, monitor, stool",
     "furniture": [
-      "arcade machine",
-      "crane game",
+      "arcade_cabinet",
       "stool",
-      "change machine"
+      "game_machine"
     ],
     "colors": [
-      "neon purple",
-      "cyan",
-      "magenta",
-      "dark blue"
+      "purple",
+      "neon_blue",
+      "red",
+      "dark"
     ],
-    "lighting": "dimly lit, cinematic lighting, neon glow, screen light, bloom",
-    "theme": "gaming, retro, cyberpunk, nightlife"
+    "lighting": "dimly_lit, ceiling_light, bloom, neon_lights",
+    "theme": "retro_gaming, cyberpunk, 80s"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/Arcade_-_Illustrious.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "Arcade_-_Illustrious"
+    "lora_weight": 0.7,
+    "lora_triggers": "4rc4d3"
   },
   "tags": [
-    "video games",
-    "amusement",
-    "electronic",
-    "nostalgia"
+    "indoors",
+    "arcade_cabinet",
+    "joystick",
+    "monitor",
+    "checkered_floor",
+    "ceiling_light",
+    "neon_lights",
+    "retro_artstyle"
   ]
 }