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/glass_block_wall_il_2_d16.json

@@ -1,36 +1,30 @@
 {
   "scene_id": "glass_block_wall_il_2_d16",
   "scene_name": "Glass Block Wall Il 2 D16",
-  "description": "A bright, retro-modern interior space featuring a textured glass block wall that distorts sunlight into soft, geometric patterns across the tiled floor.",
+  "description": "A modern indoor space featuring a distinctive glass block wall (Luxfery) that diffuses light, creating a soft, translucent geometric backdrop often paired with simple flooring.",
   "scene": {
-    "background": "glass_block_wall, indoors, architecture, white_wall, window, tiled_floor, blurred_background",
-    "foreground": "potted_plant, sunlight, heavy_shadows, no_humans, reflection",
-    "furniture": [
-      "sleek_sofa",
-      "metal_coffee_table",
-      "potted_fern"
-    ],
+    "background": "glass_wall, wall, indoors, blurry_background, simple_background",
+    "foreground": "wooden_floor",
+    "furniture": [],
     "colors": [
-      "white",
-      "cyan",
-      "light_gray",
-      "green"
+      "green_theme",
+      "grey_theme",
+      "white_theme"
     ],
-    "lighting": "natural_light, caustics, volumetric_lighting, subsurface_scattering, day",
-    "theme": "interior_design, 1980s_style, aesthetic, minimalism"
+    "lighting": "backlighting",
+    "theme": "simple_background"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/glass_block_wall_il_2_d16.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "glass_block_wall_il_2_d16"
+    "lora_triggers": "glass block wall"
   },
   "tags": [
-    "glass_block_wall",
-    "scenery",
-    "interior",
-    "translucent",
-    "retro",
-    "distortion",
-    "architectural"
+    "glass_wall",
+    "indoors",
+    "backlighting",
+    "wooden_floor",
+    "blurry_background",
+    "wall"
   ]
 }