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/checkpoints/illustrious_miaomiaoharem_v19.json

@@ -0,0 +1,10 @@
+{
+  "checkpoint_path": "Illustrious/miaomiaoHarem_v19.safetensors",
+  "checkpoint_name": "miaomiaoHarem_v19.safetensors",
+  "base_positive": "masterpiece, best quality, absurdres, newest, very aesthetic, amazing quality, highres, sensitive, ultra detailed, best anatomy, HDR, 8K, high detail RAW color art, high contrast",
+  "base_negative": "(bad hands,mutated hands and fingers:1.2), lowres, (bad), bad feet, text, error, fewer, extra, missing, worst quality, jpeg artifacts, low quality, watermark, unfinished, displeasing, oldest, early, signature, artistic error, username, scan, [abstract], english text, shiny hair, 3d, realistic",
+  "steps": 30,
+  "cfg": 5.0,
+  "sampler_name": "euler_ancestral",
+  "vae": "integrated"
+}