feat(openrouter): cap max_tokens per request (OPENROUTER_MAX_TOKENS)

OpenRouter reserves credit against the requested max_tokens, falling
back to the model's full output window when omitted, so expensive
models failed with credit-limit errors even for short replies. Default
the cap to 16384; OPENROUTER_MAX_TOKENS overrides it and 0 disables it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -52,6 +52,10 @@ AGENTIC_CHAT_MAX_ITERATIONS=6
 # OPENROUTER_EMBEDDING_MODEL=openai/text-embedding-3-small
 # OPENROUTER_HTTP_REFERER=https://your-site.example
 # OPENROUTER_APP_TITLE=ImageApi
+# Cap on generated tokens per call (sent as max_tokens). OpenRouter reserves
+# credit against the requested max, so leaving it uncapped makes pricey
+# models fail with credit-limit errors. Default 16384; 0 = no cap.
+# OPENROUTER_MAX_TOKENS=16384
 
 # ── AI Insights — local backend switch ──────────────────────────────────
 # Picks which local LLM stack the server uses for chat, vision describe,