Unified NL search Phase 1: NL→structured-query translator + forward geocoding

Foundation for the /photos/search/unified endpoint (Phase 2). Two new,
fully unit-tested pieces, not yet wired into a route (allow-until-wired,
mirroring llm_client.rs):

- ai/nl_query.rs: translate a free-text query into a StructuredQuery via one
  grounded LLM call. Two-stage — the model emits names/ISO dates, then a pure
  resolve step maps tag names against the real vocab and converts dates to
  unix seconds. Hallucinated (non-vocab) tags are surfaced in unmatched_tags
  rather than silently used as hard filters — the anti-noise guard. 12 tests.

- geo::forward_geocode + bbox_to_circle: resolve a place name to a circle via
  Nominatim /search, collapsing the bounding box to centroid + circumscribing
  radius so "Portland" and "Italy" both map onto the existing gps circle
  filter with no schema change. Radius is the max centroid-to-corner distance
  (corners aren't equidistant on a sphere). 4 tests.

fmt + clippy clean; 19 new tests pass.

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

src/ai/mod.rs

@@ -10,6 +10,7 @@ pub mod insight_generator;
 pub mod llamacpp;
 pub mod llm_client;
 pub mod local_llm;
+pub mod nl_query;
 pub mod ollama;
 pub mod openrouter;
 pub mod pronunciation;