fix(lora-worker): default network_alpha to dim/2 to match proven Sindra runs

comfy-dev cross-check: Sindra v1/v2 used alpha=dim/2 (0.5 LoRA scaling),
which produced the validated likeness; the initial alpha=dim (1.0) was a
stronger, unvalidated default. Align the default to the proven value
(operator/per-request can still override). Tests updated (14 green).

services/lora-training-worker/worker/config.py

@@ -76,7 +76,10 @@ SDXL_HPARAMS = {
     "lr_warmup_steps": "0",
     "train_batch_size_lean": "1",  # 3090
     "train_batch_size_full": "2",  # A6000
-    "network_alpha_ratio": 1.0,  # alpha = dim * ratio
+    # alpha = dim * ratio. 0.5 (alpha=dim/2) matches the PROVEN Sindra v1/v2 runs (comfy-dev
+    # cross-check 2026-07-06) — it's the scaling that produced the validated likeness. alpha=dim
+    # (1.0) is a valid stronger default but wasn't what Sindra used; operator can override.
+    "network_alpha_ratio": 0.5,
     "min_snr_gamma": "5",
     "noise_offset": "0.1",
     "save_precision": "bf16",