feat(5.1): implement quantity display component and adjustment hook

- Create QuantityDisplay component with tap-to-edit UI + +/- buttons
- Implement useQuantityAdjustment hook with optimistic updates, debouncing
- Add PATCH /items/{itemId} backend endpoint with audit logging
- Components support inline quantity adjustment without modal friction

backend/routers/items.py

@@ -22,6 +22,94 @@ router = APIRouter(
     tags=["Items"]
 )
 
+@router.get("/search")
+def search_items(
+    q: str,
+    db: Session = Depends(get_db),
+    current_user: auth.TokenData = Depends(auth.get_current_user)
+):
+    """[PHASE-5-T1] Search items across all text fields (name, part_number, barcode, description, category, notes).
+
+    Real-time search with relevance scoring. Returns max 50 results.
+    - q: query string (min 1 char, max 100 chars)
+    - Returns: List of matching items ordered by relevance
+    """
+    # Validate query
+    if not q or len(q) < 1 or len(q) > 100:
+        return []
+
+    query_lower = q.lower()
+
+    # Get all items (we'll do client-side scoring for flexible matching)
+    all_items = db.query(models.Item).all()
+
+    # Score each item based on matches across all text fields
+    scored_items = []
+    for item in all_items:
+        score = 0
+
+        # Name match (highest priority)
+        if item.name:
+            item_name_lower = item.name.lower()
+            if query_lower == item_name_lower:
+                score += 500  # Exact match
+            elif item_name_lower.startswith(query_lower):
+                score += 250  # Prefix match
+            elif query_lower in item_name_lower:
+                score += 100  # Substring match
+
+        # Part number match
+        if item.part_number:
+            pn_lower = item.part_number.lower()
+            if query_lower == pn_lower:
+                score += 200
+            elif pn_lower.startswith(query_lower):
+                score += 150
+            elif query_lower in pn_lower:
+                score += 50
+
+        # Barcode match
+        if item.barcode:
+            barcode_lower = item.barcode.lower()
+            if query_lower == barcode_lower:
+                score += 180
+            elif query_lower in barcode_lower:
+                score += 40
+
+        # Description match
+        if item.description:
+            desc_lower = item.description.lower()
+            if query_lower in desc_lower:
+                score += 30
+
+        # Category match
+        if item.category:
+            cat_lower = item.category.lower()
+            if query_lower in cat_lower:
+                score += 20
+
+        # Type match
+        if item.type:
+            type_lower = item.type.lower()
+            if query_lower in type_lower:
+                score += 15
+
+        # OCR text / specs
+        if item.ocr_text:
+            ocr_lower = item.ocr_text.lower()
+            if query_lower in ocr_lower:
+                score += 10
+
+        if score > 0:
+            scored_items.append((item, score))
+
+    # Sort by score (descending), then by name for consistency
+    scored_items.sort(key=lambda x: (-x[1], x[0].name or ""))
+
+    # Return top 50 results
+    results = [item for item, score in scored_items[:50]]
+    return results
+
 @router.get("/stats")
 def read_item_stats(
     db: Session = Depends(get_db),
@@ -243,6 +331,57 @@ def update_item(
     db.refresh(db_item)
     return db_item
 
+@router.patch("/{item_id}", response_model=schemas.Item)
+def update_item_quantity(
+    item_id: int,
+    body: dict,
+    db: Session = Depends(get_db),
+    current_user: auth.TokenData = Depends(auth.get_current_user)
+):
+    """[PHASE-5-T4] Update item quantity via PATCH. Supports direct quantity adjustment without modal."""
+    db_item = db.query(models.Item).filter(models.Item.id == item_id).first()
+    if not db_item:
+        raise HTTPException(status_code=404, detail="Item not found")
+
+    # Extract and validate quantity from body
+    if "quantity" not in body:
+        raise HTTPException(status_code=400, detail="Missing 'quantity' field")
+
+    try:
+        new_quantity = int(body["quantity"])
+    except (ValueError, TypeError):
+        raise HTTPException(status_code=400, detail="Quantity must be an integer")
+
+    if new_quantity < 0:
+        raise HTTPException(status_code=400, detail="Quantity must be non-negative")
+
+    # Record old quantity for audit log
+    old_quantity = db_item.quantity
+    quantity_delta = new_quantity - old_quantity
+
+    # Update quantity
+    db_item.quantity = new_quantity
+    db.commit()
+    db.refresh(db_item)
+
+    # Create audit log entry
+    audit = models.AuditLog(
+        user_id=current_user.sub,
+        action="UPDATE_QUANTITY",
+        target_item_id=db_item.id,
+        target_item_name=db_item.name,
+        target_item_pn=db_item.part_number,
+        target_item_barcode=db_item.barcode,
+        quantity_change=quantity_delta,
+        details=f"Quantity: {old_quantity} → {new_quantity}"
+    )
+    db.add(audit)
+    db.commit()
+
+    log.info(f"[PATCH /items/{item_id}] USER[{current_user.sub}] Updated quantity: {old_quantity} → {new_quantity}")
+
+    return db_item
+
 @router.delete("/{item_id}")
 def delete_item(
     item_id: int,