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/scenes/homeless_ossan_v2.json

@@ -1,39 +1,32 @@
 {
   "scene_id": "homeless_ossan_v2",
   "scene_name": "Homeless Ossan V2",
-  "description": "A gritty and melancholic urban scene featuring a disheveled middle-aged man sitting in a dimly lit alleyway surrounded by the remnants of daily life.",
+  "description": "A cramped and chaotic makeshift living space, characterized by urban decay and accumulation of refuse. The area is enclosed by blue tarpaulins and battered partitions, filled with scattered trash, cardboard boxes, and worn-out bedding like tatami mats and futons, illuminated by dim, dappled sunlight filtering through gaps.",
   "scene": {
-    "background": "outdoors, alley, night, brick wall, graffiti, trash, garbage, asphalt, damp, scenery, depth of field",
-    "foreground": "1male, solo, ossan, middle-aged man, beard, stubble, wrinkled skin, dirty face, messy hair, tired eyes, worn clothes, torn jacket, beanie, sitting on ground, crouching, gloom, realistic",
+    "background": "ruins, messy_room, tarpaulin, wall, partition",
+    "foreground": "trash, litter, cardboard_box, plastic_bag, crumpled_paper, mold",
     "furniture": [
-      "cardboard box",
-      "blue tarp",
-      "milk crate",
-      "empty can"
+      "tatami",
+      "futon",
+      "pillow"
     ],
     "colors": [
-      "grey",
+      "blue",
       "brown",
-      "black",
-      "mustard yellow",
-      "desaturated"
+      "grey"
     ],
-    "lighting": "dim lighting, cinematic lighting, street light, harsh shadows, low key, volumetric lighting",
-    "theme": "poverty, homelessness, urban decay, depression, survival, realistic"
+    "lighting": "dim_lighting, dappled_sunlight, light_particles",
+    "theme": "poverty, dirty, urban_decay, deterioration"
   },
   "lora": {
     "lora_name": "Illustrious/Backgrounds/homeless_ossan_V2.safetensors",
-    "lora_weight": 1.0,
-    "lora_triggers": "homeless_ossan_V2"
+    "lora_weight": 0.8,
+    "lora_triggers": "homeless_ossan"
   },
   "tags": [
-    "homeless",
-    "ossan",
-    "poverty",
-    "urban",
-    "gritty",
-    "middle-aged",
-    "realistic",
-    "depressed"
+    "dutch_angle",
+    "fisheye",
+    "scenery",
+    "no_humans"
   ]
 }