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/midis_largeroom_v0_5_il_.json

@@ -1,34 +1,35 @@
 {
   "scene_id": "midis_largeroom_v0_5_il_",
   "scene_name": "Midis Largeroom V0 5 Il ",
-  "description": "An expansive, modern interior space featuring high ceilings and panoramic floor-to-ceiling windows.",
+  "description": "A vast and distinctively large modern living room, emphasizing the wide open space and architectural details.",
   "scene": {
-    "background": "indoors, window, cityscape, sky, high_ceiling, glass_wall, scenery",
-    "foreground": "spacious, empty_room, polished_floor, depth_of_field",
+    "background": "living_room, indoors, window, scenery, architecture, ceiling",
+    "foreground": "carpet, floor",
     "furniture": [
-      "modern_sofa",
-      "coffee_table",
+      "couch",
+      "table",
+      "bookshelf",
       "potted_plant",
-      "carpet"
+      "television"
     ],
     "colors": [
       "white",
-      "beige",
-      "grey"
+      "brown",
+      "beige"
     ],
-    "lighting": "natural_light, sunlight, cinematic_lighting, soft_shadows",
-    "theme": "modern_architecture, luxury, minimalism"
+    "lighting": "sunlight, day",
+    "theme": "spacious_interior"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/midis_LargeRoom_V0.5[IL].safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "midis_LargeRoom_V0.5[IL]"
+    "lora_triggers": "largeroom"
   },
   "tags": [
-    "best_quality",
     "masterpiece",
-    "ultra_detailed",
-    "architectural_photography",
-    "no_humans"
+    "best_quality",
+    "absurdres",
+    "wide_shot",
+    "very_aesthetic"
   ]
 }