fix(backtest): bound every log tail to the final 256 KB

[Marinski]

What

The live /tail endpoint read entire log files into memory on every poll: _tail_dir_log and get_tail's run.log block called a whole-file reader, then decoded and split the result.

A Strategy Tester log grows to gigabytes while the run being polled writes it, so each poll allocated the file + a decoded copy + a line list — measured 45–65 s per call against a 1.9 GB log, with the GIL held through the decode. Every thread in the process stalled behind it: /ping took 25–50 s, the container healthcheck flapped, clients timed out, and a healthy terminal was indistinguishable from a wedged one. _tail_terminal_log (the failure-path tail) had the same unbounded read.

Fix

• _read_tail_text: seek to the final 256 KB, 2-byte aligned so UTF-16 code units stay intact; sniff UTF-16 from a BOM or NUL second byte; drop the truncated first line of a mid-file window. Used by both tail helpers and the run.log block.
• _tail_dir_log inherits the two picker rules _tail_terminal_log already had and it predated: newest by mtime, never metaeditor.log (the alphabetical pick returned a stale compile log — 'm' > '2' — instead of the run being polled).
• Whole-file reads remain only where the whole file is genuinely needed (INI parse, report parse).

Tests

12 new tests: bounded-window correctness (truncated first line dropped, UTF-16 alignment via a non-ASCII log, small-file passthrough, missing file), bounded _tail_terminal_log, and _tail_dir_log's mtime pick / metaeditor exclusion / bounded read / empty dir. Full suite: 432 passed.

[Marinski]

Folding this into #18 rather than asking you to review it separately, and closing it here.

Both change mt5api/backtest/handler.py, both are terminal-log and symbol-cache robustness, and #18 already has a review round from you on it. Two PRs over one file was one more than you needed.

The commit is now on fix/symbol-suffix-and-operator-scripts as b24399d, unchanged, and the merge was clean. #18's description covers it. The reasoning above stays here for reference.

Nothing here has been reviewed yet, so nothing is being taken away from you — if you would rather it stayed separate, say so and I will reopen it.