Improve OCR preprocessing and amount extraction robustness

Image preprocessing (receipt_parser.py):
- Add ImageOps.exif_transpose() — fixes portrait photos stored with EXIF
  rotation metadata (most phone photos); without this Tesseract reads a
  rotated image and produces garbage
- Upscale images < 600px wide for better character recognition
- Raise binarization threshold 140→160 for faint thermal-print receipts
- Try PSM 6 (single text block) before PSM 4, PSM 11 as fallbacks;
  PSM 6 is better suited to single-column receipt layout

Amount extraction (expenses_agent.py):
- Add Pass 2 bottom-of-receipt line scan when labeled Total: regex fails;
  reads lines bottom-to-top in the last 50% of text, skipping change/tip
  lines — handles 'T0TAL' OCR misread and amount-on-next-line layout
- Add _SKIP_LINE_RE and _ANY_DOLLAR_RE module-level patterns
- 8 new tests covering garbled total, change-skip, USD suffix, etc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_expenses_agent.py

@@ -458,6 +458,27 @@ class TestExtractAmount:
     def test_comma_in_amount(self):
         assert _extract_amount_from_text('Grand Total: $1,234.56') == 1234.56
 
+    def test_bottom_scan_garbled_total(self):
+        # OCR garbled "TOTAL" — bottom-scan fallback should find the amount
+        text = 'Burger  5.99\nFries   2.50\nT0TAL   8.49'
+        assert _extract_amount_from_text(text) == 8.49
+
+    def test_bottom_scan_skips_change(self):
+        # Should return the total (8.49), not the change (1.51)
+        text = 'TOTAL  8.49\nCash  10.00\nChange  1.51'
+        assert _extract_amount_from_text(text) == 8.49
+
+    def test_bottom_scan_amount_on_own_line(self):
+        # Amount printed on a separate line below the label
+        text = 'Items  5.00\nTax 0.50\nTotal\n5.50'
+        assert _extract_amount_from_text(text) == 5.50
+
+    def test_amount_due_with_usd_suffix(self):
+        # PDF text may include "USD" after the number — regex should still work
+        # via the bottom scan since the labeled-total regex won't match "USD"
+        text = 'Total Charged: $198.40 USD'
+        assert _extract_amount_from_text(text) == 198.40
+
 
 class TestExtractDate:
     def test_iso_format(self):