i18n: stop truncating IT translations + localise the chat sidebar

Three connected fixes after the user spotted the 2026-05-28 IT log
cutting off mid-sentence:

1. translation: bump max_tokens 4000 → 8000.
   call_llm()'s default cap was 4000, which is what the English log
   generator itself uses as its ceiling. Italian expands roughly 15-25 %
   over English in tokens, so any near-cap English source produced an
   IT translation that hit finish_reason=length and returned a
   truncated body — silently, because _call_provider() only raises when
   content is fully empty. The strategic_log_translations table has
   dozens of rows where completion_tokens landed at exactly 4000 with
   content well under half the source length. 8000 gives ample
   headroom for any of the five LANGUAGES we ship (en/it/es/fr/de).

2. log.html: localise the chat sidebar strings.
   user_lang was already passed into the template by pages.py, so an
   inline {% if user_lang == 'it' %} keeps it simple. Covers the
   "Ask Cassandra" title, the "grounded on…" hint, the helper lede,
   the textarea placeholder, and the Send button label.

3. chat endpoint: append respond_in_clause(user.lang) to the system
   prompt. The chat conversation can now happen in IT — the model's
   first reply lands in the right language even when the user's first
   turn is short.

scripts/backfill_truncated_translations.py: one-off cleanup utility.
Scans strategic_log_translations for rows whose translated content is
< 70 % of the English source (the truncation signal — IT *expands*
beyond English, so a shorter translation is always suspect), deletes
them, and re-translates via the now-uncapped service. Supports --date,
--since, --all and --dry-run. The 2026-05-28 fan-out has already been
re-translated (13/13 rows). Other historical dates still hold older
truncations; the user can decide whether to backfill those (the script
is idempotent).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/routers/chat.py

@@ -21,6 +21,7 @@ from app.db import get_session, utcnow
 from app.jobs._market_context import REFERENCE_LINE
 from app.models import AICall, Headline, Quote, StrategicLog
 from app.routers.api import _md_to_html
+from app.services.i18n import respond_in_clause
 from app.services.llm_prompts import build_chat_system_prompt
 from app.services.openrouter import call_llm, month_start
 
@@ -160,6 +161,13 @@ async def chat(
         headlines=headlines,
         reference_line=REFERENCE_LINE,
     )
+    # Respect the user's interface language preference: append a single
+    # localized "respond in" nudge so the assistant answers in IT when
+    # the user has lang=it. The prompt + history (which includes the
+    # user's own question, often in their language) are usually enough,
+    # but the nudge guarantees the first reply lands correctly.
+    user_lang = principal.user.lang if principal and principal.user else "en"
+    system_prompt = system_prompt + respond_in_clause(user_lang)
 
     msgs = [{"role": "system", "content": system_prompt}]
     for m in history: