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/retrokitchenill.json

@@ -1,36 +1,44 @@
 {
   "scene_id": "retrokitchenill",
   "scene_name": "Retrokitchenill",
-  "description": "A charming, nostalgic mid-century modern kitchen illustration featuring pastel colors, checkered flooring, and vintage appliances.",
+  "description": "A charming 1950s-style kitchen scene featuring pastel appliances and checkered floors, evoking a nostalgic diner atmosphere.",
   "scene": {
-    "background": "indoors, kitchen, tile floor, checkered floor, window, curtains, wallpaper, cabinets, retro style, scenery, no humans",
-    "foreground": "table, chair, refrigerator, stove, oven, sink, countertop, plant, vase, fruit bowl",
+    "background": "kitchen, indoors, checkered_floor, window",
+    "foreground": "table, chair",
     "furniture": [
-      "chrome dinette set",
-      "rounded refrigerator",
-      "gas stove",
-      "formica table"
+      "refrigerator",
+      "stove",
+      "cabinet",
+      "sink",
+      "cupboard"
     ],
     "colors": [
-      "mint green",
-      "pastel pink",
-      "cream",
-      "turquoise",
-      "chrome"
+      "pastel_colors",
+      "white"
     ],
-    "lighting": "soft lighting, natural light, flat lighting",
-    "theme": "vintage, retro, 1950s, pop art, domestic"
+    "lighting": "sunlight",
+    "theme": "1950s_(style), retro_artstyle"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/RetroKitchenILL.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "RetroKitchenILL"
+    "lora_triggers": "retro kitchens"
   },
   "tags": [
-    "retro",
-    "illustration",
     "kitchen",
-    "aesthetic",
-    "vintage"
+    "indoors",
+    "checkered_floor",
+    "refrigerator",
+    "stove",
+    "cabinet",
+    "sink",
+    "pastel_colors",
+    "1950s_(style)",
+    "no_humans",
+    "scenery",
+    "table",
+    "chair",
+    "window",
+    "sunlight"
   ]
 }