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/laboratory___il.json

@@ -1,33 +1,50 @@
 {
   "scene_id": "laboratory___il",
   "scene_name": "Laboratory Il",
-  "description": "A detailed modern science laboratory scene featuring a cluttered workbench with various chemical experiments and high-tech equipment.",
+  "description": "A detailed scientific laboratory environment featuring high-tech equipment, glassware, and sterile surfaces.",
   "scene": {
-    "background": "indoors, laboratory, science, shelves, cabinets, poster, whiteboard, machinery, glass_wall",
-    "foreground": "table, desk, beaker, flask, test_tube, liquid, chemical, microscope, computer, laptop, messy_desk, paperwork, wire",
+    "background": "indoors, laboratory, reflective_floor, science_fiction",
+    "foreground": "test_tube, vial, bottle, round-bottom_flask, chemicals, liquid, potion, paper, cable",
     "furniture": [
-      "lab_bench",
-      "metal_cabinet",
-      "stool"
+      "cabinet",
+      "shelf",
+      "monitor",
+      "table",
+      "stasis_tank"
     ],
     "colors": [
       "white",
-      "silver",
-      "cyan",
-      "glass"
+      "blue",
+      "silver"
     ],
-    "lighting": "fluorescent, bright, cold_lighting, screen_glow",
-    "theme": "science, research, chemistry, technology"
+    "lighting": "ceiling_light, artificial_lighting",
+    "theme": "science, chemistry, medical, sci-fi"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/Laboratory_-_IL.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "Laboratory_-_IL"
+    "lora_weight": 0.7,
+    "lora_triggers": "l4b"
   },
   "tags": [
-    "scenery",
-    "no_humans",
-    "hyperdetailed",
-    "realistic"
+    "indoors",
+    "laboratory",
+    "science",
+    "test_tube",
+    "monitor",
+    "shelf",
+    "vial",
+    "bottle",
+    "round-bottom_flask",
+    "cabinet",
+    "paper",
+    "reflective_floor",
+    "cable",
+    "chemicals",
+    "jar",
+    "ceiling_light",
+    "liquid",
+    "potion",
+    "science_fiction",
+    "stasis_tank"
   ]
 }