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/before_the_chalkboard_il.json

@@ -1,34 +1,40 @@
 {
   "scene_id": "before_the_chalkboard_il",
   "scene_name": "Before The Chalkboard Il",
-  "description": "A character standing confidently in a classroom setting with a chalkboard full of equations and notes in the background.",
+  "description": "A classroom scene where a character stands near the teacher's desk (lectern) in front of the chalkboard, typically used for self-introductions or announcements.",
   "scene": {
-    "background": "indoors, classroom, school, chalkboard, greenboard, math, formula, writing on board, window, daylight, dust motes",
-    "foreground": "solo, standing, looking at viewer, holding chalk, pointing, school uniform, upper body",
+    "background": "classroom, chalkboard, indoors, clock",
+    "foreground": "standing, desk, lectern, chalk",
     "furniture": [
+      "desk",
+      "lectern",
+      "school_desk",
       "chalkboard",
-      "teacher's desk",
-      "podium"
+      "clock"
     ],
     "colors": [
-      "dark green",
-      "chalk white",
-      "wood brown",
-      "creamy beige"
+      "green",
+      "brown",
+      "white"
     ],
-    "lighting": "natural light, sunbeams, soft indoor lighting, cinematic lighting",
-    "theme": "education, school life, presentation"
+    "lighting": "indoors",
+    "theme": "school, introduction"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/Before_the_Chalkboard_IL.safetensors",
     "lora_weight": 1.0,
-    "lora_triggers": "Before_the_Chalkboard_IL"
+    "lora_triggers": "standing beside desk, chalkboard, indoors"
   },
   "tags": [
     "classroom",
     "chalkboard",
-    "school",
-    "teaching",
-    "presentation"
+    "desk",
+    "lectern",
+    "school_desk",
+    "chalk",
+    "clock",
+    "indoors",
+    "standing",
+    "school_uniform"
   ]
 }