fix: correct Gemini model name (gemini-2.0-flash, not 3.1)

gemini-3.1-flash does not exist — Google's naming is 1.5/2.0/2.5.
All LLM explain/summary calls were returning reason:"error" because
the API 404'd on the model id.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.py

@@ -32,7 +32,7 @@ WIKI_HEADERS = {
 
 # ── Search ───────────────────────────────────────────────────────────────
 MAX_HOPS            = 100    # enforced as expansion-count cap, not path-length cap (see search.py)
-TOP_DISPLAY         = 25    # max neighbours sent to frontend per step
+TOP_DISPLAY         = 35    # max neighbours sent to frontend per step
 
 # ── Scoring ──────────────────────────────────────────────────────────────
 # Architect plan (Honest Bidirectional Meeting-Check Rewrite): the target's
@@ -148,7 +148,7 @@ HETIONET_URL        = ("https://github.com/hetio/hetionet/raw/main/"
 # The key is a backend-only secret — never committed, never sent to the
 # browser (only /api/llm/status's booleans are). See core/llm.py.
 GEMINI_API_KEY      = os.getenv("GEMINI_API_KEY", "")
-GEMINI_MODEL        = os.getenv("GEMINI_MODEL", "gemini-3.1-flash")
+GEMINI_MODEL        = os.getenv("GEMINI_MODEL", "gemini-2.0-flash")
 GEMINI_RPM          = int(os.getenv("GEMINI_RPM", "12"))          # soft calls/min budget — stay under the free-tier limit
 GEMINI_COOLDOWN_S   = int(os.getenv("GEMINI_COOLDOWN_S", "60"))   # pause after an upstream 429/5xx
 GEMINI_TIMEOUT_S    = float(os.getenv("GEMINI_TIMEOUT_S", "12"))