tune(translation): set latin acceptance floor to 0.80 (#1376)

Calibrated against fresh probes once Interpreter returned real langdetect
confidence: genuine German detected at 1.0, a correctly-detected but ambiguous
latin string at 0.86. Set _MIN_LATIN_CONFIDENCE to 0.80 (below that band) so
legitimate ambiguous non-English still translates while genuinely-unsure guesses
are rejected. Real langdetect also fixed the original mis-flag at the source, so
this floor is a safety net, not the primary fix. Pin 0.86-accepted in the gate
test to guard against bumping the floor back up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgZP9v2otxVJymiYsnVuMy

backend/app/tasks/translation.py

@@ -58,9 +58,16 @@ _INTERRUPT_BACKOFF_MAX = 900
 # correctly translated). Latin-script detection is a real statistical
 # probability, so a short English title Interpreter mis-labels as a European
 # language scores low; require it to clear this floor, else keep the original.
-# Tune the floor against real titles via the Settings "Test translation" box.
+# Calibrated 2026-07-09 against fresh (uncached) probes once Interpreter returned
+# real langdetect confidence: genuine German detected at 1.0, while a correctly-
+# detected but ambiguous latin string dipped to 0.86 — so the floor sits at 0.80,
+# below that band, to keep legitimate ambiguous non-English while still rejecting
+# genuinely-unsure guesses. Real langdetect also fixed the original mis-flag at
+# the source (short English titles now detect as English → passthrough), so this
+# floor is a safety net, not the primary fix. Re-tune via the "Test translation"
+# box (send fresh text — cache hits report 1.0).
 _CJK_LANGS = frozenset({"ja", "ko", "zh"})
-_MIN_LATIN_CONFIDENCE = 0.90
+_MIN_LATIN_CONFIDENCE = 0.80
 
 
 def _interrupt_backoff(retry_after) -> int:

tests/test_translation_gate.py

@@ -17,6 +17,7 @@ def test_accept_trusts_cjk_regardless_of_confidence():
 
 def test_accept_requires_floor_for_latin():
     assert _accept("de", 0.98) is True
+    assert _accept("de", 0.86) is True    # calibrated: ambiguous-but-correct German stays
     assert _accept("de", _MIN_LATIN_CONFIDENCE) is True          # exactly at floor
     assert _accept("de", _MIN_LATIN_CONFIDENCE - 0.01) is False
     assert _accept("fr", 0.40) is False                          # short mis-flag