mAi: #10 - multi-model backend expansion (workflow templates + compare harness)

Path 1 architecture: one comfyui adapter, workflows as data.

- workflow_template.go: embed.FS + token substitution with type-preserving
  whole-value placeholders. ${prompt} → string, ${seed} → int64,
  ${cfg} → float64 — no JSON round-tripping. Partial matches ignored.
- comfyui.go: refactored to load workflow from embedded FS or filesystem
  path. Back-compat preserved: workflow: defaults to flux1-schnell.
- workflows/{flux1-schnell,flux2-klein,sd35-medium}.json — bundled
  templates. flux1-schnell migrated from hardcoded with identical node IDs.
- compare.go: new `imagen compare` subcommand. Sequential N-backend run
  (one GPU on mRock — parallel would OOM), per-backend PNG, sidecar JSON
  with per-model metadata + errors, composite contact sheet via Go image
  package (no ImageMagick dep).
- Sample config gains flux2-klein-local + sd35-medium-local instances.
- docs/backends.md: architecture rationale + per-model HF download paths
  + how to add a new bundled workflow + compare-harness reference.

Live smoke verified: compare mock + flux-schnell-local at 768×768 →
both PNGs written, sidecar JSON has workflow="flux1-schnell" + full
metadata, contact sheet renders. Worker contract (Request → Generate)
unchanged, so flexsiebels /imagine UI API surface preserved.

Tests: 11 existing comfyui + 6 new workflow_template + 5 new compare
tests, all green.

Adding a new model is now yaml + JSON, never Go.

internal/config/config.go

@@ -144,15 +144,54 @@ output:
   cloud_sync: auto
 
 backends:
+  # FLUX.1-schnell on the local ComfyUI server. The "workflow" key picks the
+  # bundled template under internal/backend/workflows/; omit it for back-compat
+  # (defaults to flux1-schnell). See docs/backends.md for the per-model setup.
   flux-schnell-local:
     type: comfyui
     base_url: http://mrock:8188
+    workflow: flux1-schnell
     # Filename of the unet checkpoint inside the ComfyUI server's
-    # models/unet/ directory. See docs/setup-comfyui-mrock.md.
+    # models/unet/ directory.
     model: flux1-schnell.safetensors
+    vae: ae.safetensors
+    clip_l: clip_l.safetensors
+    clip_t5: t5xxl_fp8_e4m3fn.safetensors
+    dtype: fp8_e4m3fn
     default_steps: 4
     default_sampler: euler
     default_scheduler: simple
+    default_cfg: 1.0
+
+  # FLUX.2 [klein] 4B distilled — sub-second on RTX 4070 Ti SUPER.
+  # Weights: BFL non-commercial; flux-2-klein-base-4b-fp8 in models/unet/,
+  # qwen_3_4b in models/text_encoders/, flux2-vae in models/vae/.
+  flux2-klein-local:
+    type: comfyui
+    base_url: http://mrock:8188
+    workflow: flux2-klein
+    model: flux-2-klein-base-4b-fp8.safetensors
+    vae: flux2-vae.safetensors
+    clip: qwen_3_4b.safetensors
+    dtype: fp8_e4m3fn
+    default_steps: 4
+    default_sampler: euler
+    default_scheduler: simple
+    default_cfg: 1.0
+    guidance: 4.0
+
+  # SD3.5 medium — single-checkpoint variant that bundles the three text
+  # encoders inside the .safetensors. Drop into models/checkpoints/.
+  sd35-medium-local:
+    type: comfyui
+    base_url: http://mrock:8188
+    workflow: sd35-medium
+    model: sd3.5_medium_incl_clips_t5xxlfp8scaled.safetensors
+    default_steps: 28
+    default_sampler: dpmpp_2m
+    default_scheduler: sgm_uniform
+    default_cfg: 4.5
+    shift: 3.0
 
   mock:
     type: mock