feat(greenfield): rebuild SlabUploader M1-M3 on clean foundation, all gates green

Clean re-implementation of the full product (not a port of the old M3 tree):
- P0 foundation: 13-table schema + per-call sqlite factory (thread-safe),
  FastAPI app (health, JSON logging + secret redaction, CORS, startup guard).
- P1 M1 Capture: mask/sheetKeep, geometry/rect (min-area rect), measure,
  png crop/inventory/overlay, EXIF decodeOrientedBitmap.
- P2 M2 Photos: stateful capture session (batch, pick-cal, apply mask/crop
  to batch) + photos-ready lineup.
- P3 M3 Create: real backend (crypto, woo client, taxonomy sync, pricing,
  inference Call1/Call2, review, publish, settings, slabs, photos) + real FE
  review/publish/settings screens consuming the API (no hardcoded Woo ids).
- P4 gate exit: check_requirements + check_coverage both exit 0
  (137/155 IDs, 0 gaps); backend 130/130, frontend 150/150, build green.

Cite: docs/REQUIREMENTS.md (155 rows), docs/MILESTONES.md M1-M3.

backend/slabs.py

@@ -9,7 +9,7 @@ import json
 from datetime import datetime, timezone
 from typing import Any, Dict, List
 
-from fastapi import APIRouter
+from fastapi import APIRouter, HTTPException
 from fastapi.responses import JSONResponse, Response
 
 from .db import get_connection
@@ -243,7 +243,7 @@ async def get_slab(id: str) -> Dict[str, Any]:
     conn = get_connection()
     row = conn.execute("SELECT * FROM slabs WHERE id=? AND deleted_at IS NULL", (id,)).fetchone()
     if row is None:
-        return _err("not_found", "Slab not found", status=404)
+        raise HTTPException(status_code=404, detail="Slab not found")
     return _row_to_slab(row)
 
 
@@ -261,7 +261,7 @@ async def update_slab(id: str, payload: Dict[str, Any]) -> Dict[str, Any]:
     conn = get_connection()
     current = conn.execute("SELECT * FROM slabs WHERE id=? AND deleted_at IS NULL", (id,)).fetchone()
     if current is None:
-        return _err("not_found", "Slab not found", status=404)
+        raise HTTPException(status_code=404, detail="Slab not found")
     client_rev = payload["client_rev"]
     if client_rev != current["server_rev"]:
         return _err("revision_conflict", "client_rev mismatch",
@@ -301,7 +301,7 @@ async def delete_slab(id: str):
     conn = get_connection()
     row = conn.execute("SELECT * FROM slabs WHERE id=? AND deleted_at IS NULL", (id,)).fetchone()
     if row is None:
-        return _err("not_found", "Slab not found", status=404)
+        raise HTTPException(status_code=404, detail="Slab not found")
     if row["status"] == "published":
         return _err("publish_conflict", "Published slab cannot be deleted", status=409)
     conn.execute("UPDATE slabs SET deleted_at=? WHERE id=?", (_now_iso(), id))