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/forestill.json

@@ -1,35 +1,36 @@
 {
   "scene_id": "forestill",
   "scene_name": "Forestill",
-  "description": "A lush, enchanted forest floor illuminated by shafts of sunlight filtering through ancient trees, rendered in a detailed illustrative style.",
+  "description": "A dense, atmospheric forest scene featuring mossy rocks, trees, and a river, potentially providing a dark or dappled sunlight backdrop.",
   "scene": {
-    "background": "forest, tree, nature, outdoors, scenery, woodland, trunk, leaf, ancient tree, dense foliage, moss",
-    "foreground": "grass, flower, path, roots, fern, plant, mushroom",
-    "furniture": [
-      "fallen log",
-      "stump"
-    ],
+    "background": "forest, tree, dark_background, nature, outdoors, scenery",
+    "foreground": "grass, moss, rock, river, stream, snow",
+    "furniture": [],
     "colors": [
-      "emerald green",
-      "earth brown",
-      "golden yellow",
-      "soft teal"
+      "green",
+      "brown",
+      "dark_green"
     ],
-    "lighting": "sunlight, sunbeam, dappled sunlight, god rays, atmospheric lighting",
-    "theme": "nature, fantasy, ethereal, mysterious, tranquil"
+    "lighting": "dappled_sunlight, shadow",
+    "theme": "nature"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/ForestILL.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "ForestILL"
+    "lora_weight": 0.75,
+    "lora_triggers": "forest, tree, dark_background"
   },
   "tags": [
     "forest",
-    "illustration",
+    "tree",
+    "grass",
+    "moss",
+    "rock",
+    "river",
+    "dappled_sunlight",
+    "dark_background",
+    "snow",
     "scenery",
-    "fantasy",
-    "nature",
-    "detailed",
-    "vegetation"
+    "outdoors",
+    "nature"
   ]
 }