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/arcadev2.json

@@ -1,38 +1,36 @@
 {
   "scene_id": "arcadev2",
   "scene_name": "Arcadev2",
-  "description": "A vibrant, dimly lit retro arcade filled with rows of classic game cabinets and neon signs.",
+  "description": "A vibrant, retro-styled video arcade interior filled with rows of gaming cabinets, illuminated by neon glows and ceiling lights over a classic checkered floor.",
   "scene": {
-    "background": "indoors, arcade, row of arcade machines, arcade cabinet, neon sign, patterned carpet, posters, crowd background",
-    "foreground": "arcade machine, joystick, buttons, coin slot, reflection, glowing screen",
+    "background": "indoors",
+    "foreground": "arcade_cabinet",
     "furniture": [
-      "arcade cabinet",
-      "stool",
-      "pinball machine",
-      "crane game"
+      "arcade_cabinet",
+      "stool"
     ],
     "colors": [
-      "neon purple",
-      "neon blue",
-      "cyan",
-      "magenta",
-      "black"
+      "neon_lights",
+      "colorful"
     ],
-    "lighting": "dimly lit, neon glow, backlighting, screen glow, cinematic lighting",
-    "theme": "retro, synthwave, 1980s, gaming, cyberpunk"
+    "lighting": "ceiling_light",
+    "theme": "arcade"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/ArcadeV2.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "ArcadeV2"
+    "lora_weight": 0.7,
+    "lora_triggers": "4rc4d3"
   },
   "tags": [
     "arcade",
-    "scenery",
-    "retro gaming",
     "indoors",
-    "neon lights",
-    "video games",
-    "atmosphere"
+    "arcade_cabinet",
+    "joystick",
+    "monitor",
+    "checkered_floor",
+    "ceiling_light",
+    "neon_lights",
+    "stool",
+    "dim_lighting"
   ]
 }