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/backstage.json

@@ -1,35 +1,36 @@
 {
   "scene_id": "backstage",
   "scene_name": "Backstage",
-  "description": "A cluttered and atmospheric backstage dressing room area behind the scenes of a performance venue.",
+  "description": "A behind-the-scenes perspective of a concert or theater stage, characterized by heavy velvet curtains, industrial scaffolding, and stage equipment waiting in the wings.",
   "scene": {
-    "background": "indoors, backstage, brick wall, electrical wires, flight cases, musical equipment, posters on wall",
-    "foreground": "vanity mirror, light bulbs, makeup tools, scattered clothes, clothing rack, hanging clothes",
+    "background": "backstage, stage_curtains, scaffolding, indoors, crowd",
+    "foreground": "microphone_stand, amplifier, cable, instrument",
     "furniture": [
-      "vanity table",
-      "folding chair",
-      "clothing rack",
-      "sofa"
+      "scaffolding",
+      "microphone_stand",
+      "amplifier"
     ],
     "colors": [
-      "tungsten yellow",
-      "dark grey",
       "black",
-      "wood brown"
+      "red",
+      "grey"
     ],
-    "lighting": "dim lighting, warm lighting, artificial light, cinematic lighting, dramatic shadows",
-    "theme": "behind the scenes, concerts, theater, preparation"
+    "lighting": "stage_lights, spotlight, artificial_lighting, dimly_lit",
+    "theme": "music_concert, performance, idol, theater"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/Backstage.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "Backstage"
+    "lora_weight": 0.8,
+    "lora_triggers": "b4ckst4g3"
   },
   "tags": [
     "backstage",
-    "dressing room",
-    "green room",
-    "behind the scenes",
-    "concert venue"
+    "curtains",
+    "scaffolding",
+    "indoors",
+    "stage_curtains",
+    "stage_lights",
+    "crowd",
+    "microphone_stand"
   ]
 }