Fuzzy port-name search

Fuzzy search used by the autocomplete input on each side of the distance calculator.

Matching strategy: case-insensitive prefix match first (ILIKE 'q%'), then trigram similarity for misspellings. Results are ordered by a composite score (prefix matches always rank above pure trigram hits) so typing 'hou' surfaces 'Houston' ahead of 'South-East-Houthuizen'.

Two modes:

• From side (no from_port_id) — returns every name match, routable or not, each annotated with routable. A non-routable hit (e.g. Mumbai, which has polygons but zero cache rows because the AIS traffic lands at JNPT next door) carries nearest_routable: the closest port by great-circle that is routable, so the UI can offer "try JNPT instead" rather than letting the famous port silently disappear.
• To side (from_port_id set) — narrows to destinations actually reachable from that origin, so the user never sees a suggestion that would route_not_in_cache on click. These are all routable by construction (routable=true, nearest_routable=null).