Import Stamps Balances Trend

Upload Stamps.com's own "BalancesTrend" report -- a human export from its web dashboard (Reports tab), fetched by nobody automatically (see stamps_balances_trend.py's own module docstring for what was checked).

Frederick, 2026-09-24, having sent the report itself: "This is the vendor's own daily ledger, and it's the authoritative Stamps.com cost." Parses the file, stores the vendor's own raw daily figures (idempotent -- a second upload of the SAME file changes nothing), and OVERRIDES vendor_reported_days for every day it covers -- a deliberate correction from a more authoritative source, the one exception to the day-keeper's own "kept once, never revisited" rule. Flags any day whose report Ending Balance disagrees with Roost's own last balance reading that day.