Fix SINCE mode requests returning wrong data

When requesting candles with start date but no end date (SINCE mode),
EDM was skipping gap processing entirely because gap_end was None.
This caused requests to return cached data clipped to the most recent
N candles, ignoring the requested start date.

Fix: Normalize gaps with end=None to use current time, so SINCE mode
requests properly fetch data from the requested start time to now.

Bug symptoms:
- Request start=2026-03-01, limit=100
- Got 100 candles starting from 2026-03-04 (most recent 100)
- Expected 100 candles starting from 2026-03-01

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/exchange_data_manager/cache/manager.py

@@ -189,6 +189,20 @@ class CacheManager:
             source = "exchange" if exchange_candles else "memory"
             return result, source
 
+        # Step 2b: Handle SINCE mode (start provided, no end)
+        # Gaps with end=None need to be fetched from exchange directly
+        import time as time_module
+        now = int(time_module.time())
+        normalized_gaps = []
+        for gap_start, gap_end in gaps_to_fill:
+            if gap_start is not None and gap_end is None:
+                # SINCE mode: fetch from start to now
+                normalized_gaps.append((gap_start, now))
+            elif gap_start is not None and gap_end is not None:
+                normalized_gaps.append((gap_start, gap_end))
+            # Skip gaps where start is None (shouldn't happen with proper request)
+        gaps_to_fill = normalized_gaps if normalized_gaps else gaps_to_fill
+
         db_hit = False
         exchange_hit = False