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/canopy_bed.json

@@ -1,34 +1,35 @@
 {
   "scene_id": "canopy_bed",
   "scene_name": "Canopy Bed",
-  "description": "An ornate and elegant bedroom featuring a grand four-poster canopy bed with flowing sheer drapes, bathed in soft window light.",
+  "description": "A luxurious bedroom interior centered around a classic canopy bed with heavy draping curtains and soft bedding.",
   "scene": {
-    "background": "indoors, bedroom, master_bedroom, ornate_wallpaper, large_window, patterned_carpet, victorian_decor",
-    "foreground": "canopy_bed, four-poster_bed, bed_curtains, white_bed_sheet, plush_pillows, duvet, no_humans",
+    "background": "indoors, bedroom, curtains, window, wall",
+    "foreground": "canopy_bed, bed_sheet, pillow, white_bed_sheet",
     "furniture": [
-      "bedside_table",
-      "antique_lamp",
-      "vanity_desk",
-      "tufted_bench"
+      "canopy_bed",
+      "bed"
     ],
     "colors": [
-      "cream",
-      "gold",
       "white",
-      "pale_blue"
+      "blue"
     ],
-    "lighting": "soft_lighting, natural_light, volumetric_lighting, sunbeams",
-    "theme": "aristocratic, cozy, vintage, romance"
+    "lighting": "soft_lighting",
+    "theme": "bedroom"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/Canopy_Bed.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "Canopy_Bed"
+    "lora_weight": 0.8,
+    "lora_triggers": "c4nopy"
   },
   "tags": [
-    "interior_design",
-    "furniture",
-    "scenery",
-    "classic"
+    "indoors",
+    "bedroom",
+    "canopy_bed",
+    "curtains",
+    "pillow",
+    "bed_sheet",
+    "white_bed_sheet",
+    "soft_lighting",
+    "no_humans"
   ]
 }