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/ffm_threesome_girl_sandwichdouble_dip_illustrious.json

@@ -0,0 +1,40 @@
+{
+  "action_id": "ffm_threesome_girl_sandwichdouble_dip_illustrious",
+  "action_name": "Ffm Threesome Girl Sandwichdouble Dip Illustrious",
+  "action": {
+    "full_body": "Three-person stack on a bed: one girl lying flat on her back, the male (often faceless/obscured) positioned in the middle, and the second girl straddling on top of the pile.",
+    "head": "Girls' faces visible, often with flushed cheeks or ahegao expressions; male face usually out of frame or obscured.",
+    "eyes": "rolled_back, closed_eyes, or looking_at_viewer",
+    "arms": "Arms embracing the partner in the middle or holding bed sheets.",
+    "hands": "grabbing_sheet or touching_partner",
+    "torso": "Sandwiched torsos, breasts pressed against the middle partner.",
+    "pelvis": "Interconnected pelvises, implied penetration.",
+    "legs": "Bottom girl with legs_spread, top girl straddling.",
+    "feet": "barefoot",
+    "additional": "Scene typically set on a bed with messy sheets."
+  },
+  "lora": {
+    "lora_name": "Illustrious/Poses/FFM_threesome_girl_sandwichdouble_dip_Illustrious.safetensors",
+    "lora_weight": 1.0,
+    "lora_triggers": "ffm_threesome_double_dip"
+  },
+  "tags": [
+    "ffm_threesome",
+    "group_sex",
+    "sandwiched",
+    "2girls",
+    "1boy",
+    "girl_on_top",
+    "on_back",
+    "lying",
+    "straddling",
+    "faceless_male",
+    "male_on_top",
+    "stack",
+    "sex",
+    "implied_penetration",
+    "ahegao",
+    "bed_sheet",
+    "messy_bed"
+  ]
+}