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/tests/test_db.py

@@ -0,0 +1,40 @@
+import pytest
+import tempfile
+import os
+
+from backend.db import get_connection, init_db
+
+
+EXPECTED_TABLES = {
+    "slabs", "slab_photos", "pricing_rules", "woo_categories",
+    "woo_attributes", "woo_attribute_terms", "woo_tags", "settings",
+    "schema_version", "prompts", "prompt_revisions", "sync_log", "inference_log",
+}
+
+
+def test_init_db_creates_all_13_tables(tmp_path):
+    db_path = str(tmp_path / "test_schema.db")
+    init_db(db_path)
+    conn = get_connection(db_path)
+    rows = conn.execute("SELECT name FROM sqlite_master WHERE type='table'").fetchall()
+    names = {r["name"] for r in rows}
+    assert names >= EXPECTED_TABLES
+    assert len(EXPECTED_TABLES) == 13
+
+
+def test_slab_insert_read_roundtrip(tmp_path):
+    db_path = str(tmp_path / "test_slab.db")
+    init_db(db_path)
+    conn = get_connection(db_path)
+    now = "2026-01-01T00:00:00+00:00"
+    conn.execute(
+        """INSERT INTO slabs (id, status, sku, length_in, thickness_in, sqft, bdft,
+           client_rev, server_rev, created_at, updated_at)
+           VALUES ('slab-1', 'draft', 'SLAB-001', 10.0, 2.0, 5.0, 40.0, 1, 1, ?, ?)""",
+        (now, now),
+    )
+    conn.commit()
+    row = conn.execute("SELECT id, sku, status FROM slabs WHERE id=?", ("slab-1",)).fetchone()
+    assert row["id"] == "slab-1"
+    assert row["sku"] == "SLAB-001"
+    assert row["status"] == "draft"