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_semimergeij_ilv5vae.json

@@ -0,0 +1,10 @@
+{
+  "checkpoint_path": "Illustrious/semimergeij_ilv5vae.safetensors",
+  "checkpoint_name": "semimergeij_ilv5vae.safetensors",
+  "base_positive": "high_quality, highres, beautiful, detailed, (masterpiece, best quality:1.2), absurdres, 8k, HDR",
+  "base_negative": "worst_quality, bad_quality, poorly_detailed",
+  "steps": 30,
+  "cfg": 7.0,
+  "sampler_name": "euler_ancestral",
+  "vae": "integrated"
+}