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/nightclub.json

@@ -1,27 +1,22 @@
 {
   "scene_id": "nightclub",
   "scene_name": "Nightclub",
-  "description": "A vibrant and energetic nightclub interior pulsing with neon lasers, haze, and a crowded dance floor atmosphere.",
+  "description": "A vibrant and moody interior of a nightclub heavily styled with purple tones and neon lighting.",
   "scene": {
-    "background": "indoors, nightclub, crowd, bokeh, dj booth, shelves, alcohol bottles, wall, dark",
-    "foreground": "dance floor, confetti, haze, laser beam",
+    "background": "indoors, nightclub, purple_background, purple_theme, shelf",
+    "foreground": "dance_floor",
     "furniture": [
-      "bar counter",
-      "bar stool",
-      "disco ball",
-      "couch",
-      "speakers",
-      "amplifier"
+      "bar_counter",
+      "bar_stool",
+      "disco_ball"
     ],
     "colors": [
       "purple",
-      "cyan",
-      "neon pink",
-      "black",
-      "midnight blue"
+      "violet",
+      "black"
     ],
-    "lighting": "cinematic lighting, volumetric lighting, strobe lights, dim, colorful, backlighting",
-    "theme": "nightlife, party, edm, rave"
+    "lighting": "neon_lights, dim_lighting, stage_lights",
+    "theme": "nightlife"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/nightclub.safetensors",
@@ -30,11 +25,10 @@
   },
   "tags": [
     "nightclub",
-    "party",
-    "neon",
-    "music",
-    "dance",
-    "crowd",
-    "alcohol"
+    "purple_theme",
+    "neon_lights",
+    "indoors",
+    "bar_counter",
+    "disco_ball"
   ]
 }