Replaced the Title filter with a general search, run by the backend

GET /api/opportunities/search?q= returns the UUIDs of the matching
opportunities, and the frontend filters its list by them. Words without
a prompt search the title and description; title:, buyer:, buyer_id:
and cpv: search only their own field, and a leading ! leaves out what a
term matches. Repeats of one prompt are OR, everything else AND. CPV
terms are checked per code: cpv:72 !cpv:7221 needs a code starting 72
that doesn't start 7221.

Unknown prompts are reported, and every response carries the help that
the search box's "?" panel shows. Migration 00011 adds the description
and buyer name to the full-text index, and indexes CPV descriptions.

web/README.md

@@ -1,8 +1,9 @@
 # JobSearch - Frontend
 
 A Vite + SolidJS + Tailwind CSS (v4) + TypeScript table of opportunities with
-title search, status, mark and classification filters, and per-opportunity
-mark and hide controls.
+a general search (run by the backend, with title:, buyer:, buyer_id: and cpv:
+prompts), status, mark and classification filters, and per-opportunity mark
+and hide controls.
 
 ## Run