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/bedroom_0f_4_succubus_i.json

@@ -1,35 +1,43 @@
 {
   "scene_id": "bedroom_0f_4_succubus_i",
   "scene_name": "Bedroom 0F 4 Succubus I",
-  "description": "A lavish and moody gothic bedroom designed for a succubus, featuring dark romantic aesthetics, mystical colors, and ornate furniture.",
+  "description": "A dark and seductive bedroom designed for a succubus, featuring a grand canopy bed with red velvet, black stone walls, glowing runes, and gothic decor.",
   "scene": {
-    "background": "indoors, bedroom, gothic architecture, stone walls, stained glass window, night",
-    "foreground": "ornate canopy bed, silk sheets, heart motif, scattered rose petals, magical aura",
+    "background": "indoors, stone_wall, black_wall, runes, glowing, smoke, incense",
+    "foreground": "chain, skull",
     "furniture": [
-      "canopy bed",
-      "antique vanity",
-      "candelabra",
-      "heavy velvet curtains"
+      "canopy_bed",
+      "curtains",
+      "mirror",
+      "candle"
     ],
     "colors": [
-      "crimson",
-      "black",
-      "deep purple",
-      "gold"
+      "black_theme",
+      "red_theme"
     ],
-    "lighting": "dim lighting, candlelight, purple ambient glow, volumetric lighting",
-    "theme": "dark fantasy"
+    "lighting": "dim_lighting, candlelight",
+    "theme": "gothic_architecture, fantasy"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/bedroom_0f_4_succubus-i.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "bedroom_0f_4_succubus-i"
+    "lora_triggers": "bedroom 0f 4 succubus"
   },
   "tags": [
-    "interior",
-    "gothic",
-    "fantasy",
-    "luxurious",
-    "succubus theme"
+    "indoors",
+    "scenery",
+    "no_humans",
+    "bedroom",
+    "gothic_architecture",
+    "stone_wall",
+    "canopy_bed",
+    "runes",
+    "candlelight",
+    "red_theme",
+    "black_theme",
+    "dim_lighting",
+    "chain",
+    "skull",
+    "incense"
   ]
 }