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/il_cheekbj.json

@@ -0,0 +1,28 @@
+{
+  "action_id": "il_cheekbj",
+  "action_name": "Il Cheekbj",
+  "action": {
+    "full_body": "kneeling",
+    "head": "fellatio, cheek_bulge, open_mouth",
+    "eyes": "looking_at_viewer",
+    "arms": "hands_on_legs",
+    "hands": "resting",
+    "torso": "leaning_forward",
+    "pelvis": "kneeling",
+    "legs": "kneeling",
+    "feet": "toes_on_ground",
+    "additional": "penis"
+  },
+  "lora": {
+    "lora_name": "Illustrious/Poses/IL_cheekbj.safetensors",
+    "lora_weight": 0.8,
+    "lora_triggers": "ch33k_bj, one_cheek_bulge, fellatio"
+  },
+  "tags": [
+    "fellatio",
+    "cheek_bulge",
+    "kneeling",
+    "looking_at_viewer",
+    "penis"
+  ]
+}