search: allow a db path from the environment

Allow a db path from the environment, and refuse what it cannot read

Three changes related to opening the database:

- provide an environment override for the db path: SPINE_SEARCH_DB_PATH
  overrides the compiled-in db_sqlite_path, beside the other
  environment.get calls. Path was the one setting without an override,
  (SERVER_NAME, REQUEST_SCHEME, SCRIPT_NAME, QUERY_STRING could come
  from the environment). This adds consistency and makes testing the
  search form database builds easier (as it no longer requires
  rebuilding the binary with a patched configuration for readily
  accessible db path)

- selected_db is required to be a plain filename within the configured
  directory, the database is opened read only, and a database that is
  not there is named rather than created.

- The form now reads PRAGMA user_version, as stamped by spine, and
  refuses a database built to another schema version, naming both
  versions and the two commands that rebuild it. A database carrying
  neither of the tables it joins is refused the same way.

sqlite_db_schema_version_- is expected and has to be in sync/step with
the sqlite_db_schema_version in
sisudoc-spine src/sisudoc/outputs/io_out/sqlite.d; the two tests added
there under test/ check that they agree.

(assisted by Claude-Code)

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