Always overwrite tsdb duplicates found during backfill

Enable the previously commented-out dedupe-and-write logic in
`start_backfill()` to ensure tsdb stays clean of duplicate
entries.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/tsp/_history.py

@@ -708,8 +708,16 @@ async def start_backfill(
                     deduped,
                     diff,
                 ) = dedupe(df)
-                # if diff:
+                if diff:
-                #     sort_diff(df)
+                    log.warning(
+                        f'Found {diff} duplicates in tsdb, '
+                        f'overwriting with deduped data\n'
+                    )
+                    await storage.write_ohlcv(
+                        col_sym_key,
+                        deduped,
+                        timeframe,
+                    )
 
         else:
             # finally filled gap