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/actions/pussy_sandwich_v0_8_illu_done.json

@@ -0,0 +1,36 @@
+{
+  "action_id": "pussy_sandwich_v0_8_illu_done",
+  "action_name": "Pussy Sandwich V0 8 Illu Done",
+  "action": {
+    "full_body": "2girls, lying, cooperative_grinding, girl_on_top",
+    "head": "looking_back, looking_at_viewer",
+    "eyes": "detailed_eyes",
+    "arms": "arm_around_waist, grab",
+    "hands": "touching_partner",
+    "torso": "twisted_torso, nude, facing_away",
+    "pelvis": "ass_focus, presenting_hindquarters, tribadism",
+    "legs": "spread_legs, kneeling",
+    "feet": "barefoot",
+    "additional": "pussy_sandwich, sidelighting, deep_skin, shiny_skin"
+  },
+  "lora": {
+    "lora_name": "Illustrious/Poses/pussy_sandwich_v0.8-illu_done.safetensors",
+    "lora_weight": 1.0,
+    "lora_triggers": "pussy_sandwich, cooperative_grinding, tribadism, ass, pussy"
+  },
+  "tags": [
+    "pussy_sandwich",
+    "cooperative_grinding",
+    "tribadism",
+    "2girls",
+    "lying",
+    "girl_on_top",
+    "from_behind",
+    "looking_back",
+    "ass_focus",
+    "spread_legs",
+    "nude",
+    "masterpiece",
+    "best_quality"
+  ]
+}