Unified search: UNIFIED_SEARCH_MODEL env override for the translation step

Pin the NL->structured translation to a small, fast model that can stay
co-resident with CLIP (and the chat model) so it never evicts them on a tight
VRAM budget. Precedence: UNIFIED_SEARCH_MODEL env > client-selected model >
configured default. Logs the effective model (backend.model()) so model A/B
tests are visible. Documented in .env.example.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -80,6 +80,16 @@ AGENTIC_CHAT_MAX_ITERATIONS=6
 # LLAMA_SWAP_ALLOWED_MODELS=chat,vision,embed
 # LLAMA_SWAP_REQUEST_TIMEOUT_SECONDS=180
 
+# ── Unified search translation model (optional) ─────────────────────────
+# /photos/search/unified runs one small LLM call to translate a natural-
+# language query into structured filters + a semantic term, then CLIP-ranks.
+# That step needs an LLM AND CLIP available at once. On a tight VRAM budget a
+# large chat model can't co-reside with CLIP, so pin a small, fast model here
+# (it can stay loaded alongside CLIP and the chat model). Precedence:
+# UNIFIED_SEARCH_MODEL > the client's selected model > the configured default.
+# Use the configured backend (LLM_BACKEND); local only — no hybrid.
+# UNIFIED_SEARCH_MODEL=qwen3-0.6b
+
 # ── Text-to-speech (optional, requires LLAMA_SWAP_URL) ───────────────────
 # TTS routes through the same llama-swap proxy (a Chatterbox model id), so it
 # only needs LLAMA_SWAP_URL — it does NOT require LLM_BACKEND=llamacpp.