feat(m4): crop_fill setting + tape-line prototype (#99)

Add global crop_fill percent (default 90, 1-100) as a settings key with
FastAPI 4xx validation. Wires the value through to the inventory PNG
encode (non-retro: stored PNGs untouched). Adds a throwaway
TapePrototype.svelte behind ?tape-prototype for the #99 design sandbox.

Gates: check_requirements PASS, check_coverage PASS, settings tests 8/8.

backend/settings.py

@@ -28,13 +28,18 @@ PLAIN_KEYS: set[str] = {
     "woo_create_status",
     "taxonomy_last_sync_at",
     "taxonomy_anchor",
+    "crop_fill",
 }
 ALL_KEYS = SECRET_KEYS | PLAIN_KEYS
+# Plain keys surfaced as numbers in the API (stored as TEXT in `settings`).
+NUMERIC_KEYS: set[str] = {"crop_fill"}
 
 DEFAULTS: dict[str, Any] = {
     "inference_enabled": False,
     "content_llm_enabled": False,
     "woo_create_status": "draft",
+    # #99: global crop fill %, one source of truth for the ref line + PNG crop.
+    "crop_fill": 90,
 }
 
 
@@ -91,6 +96,12 @@ def get_settings() -> dict[str, Any]:
                 result[k] = val if val is not None else DEFAULTS.get(k)
             else:
                 result[k] = DEFAULTS.get(k)
+            if k in NUMERIC_KEYS:
+                try:
+                    f = float(result[k])
+                    result[k] = int(f) if f.is_integer() else f
+                except (TypeError, ValueError):
+                    result[k] = DEFAULTS.get(k)
 
     return result
 
@@ -108,6 +119,25 @@ def update_settings(payload: dict[str, Any]) -> dict[str, Any]:
             },
         )
 
+    # #99: rules live here in the 4xx layer, not in the DB (structural only).
+    if "crop_fill" in payload:
+        raw = payload["crop_fill"]
+        try:
+            pct = float(raw)
+        except (TypeError, ValueError):
+            pct = None
+        if pct is None or not (1 <= pct <= 100):
+            raise HTTPException(
+                status_code=400,
+                detail={
+                    "error": {
+                        "code": "validation",
+                        "message": "crop_fill must be a number between 1 and 100 (percent)",
+                        "detail": {"field": "crop_fill"},
+                    }
+                },
+            )
+
     conn = get_connection()
     for k, v in payload.items():
         if k not in ALL_KEYS: