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.

README.md

@@ -234,6 +234,7 @@ second one fails with "address already in use".
 | `internal/cli` | Cobra commands and shared CLI setup |
 | `internal/config` | Configuration loading, validation and editing |
 | `internal/fetcher` | Runs a worker per enabled source |
+| `internal/search` | Parsing of the general opportunity search (`title:`, `buyer:`, `buyer_id:`, `cpv:`, `!`) and its help |
 | `internal/skiprules` | The rules that mark new opportunities Skipped (see [internal/skiprules](internal/skiprules/README.md)) |
 | `internal/store` | SQLite storage of opportunities, and its migrations (see [internal/store](internal/store/README.md)) |
 | `internal/findatender` | FTS API client and conversion from OCDS to JobSearch types |