DocuBrowse v1.5.1 — Korean/Japanese search bug fixes

Bug-fix release resolving CJK (Korean/Japanese) search and language-configuration
issues found after v1.5.0. No schema changes; switching the embedder/tokenizer
language still requires a re-scan.

Fixes

• Semantic search no longer returns HTTP 500 after a language switch.
  Semantic scoring now compares only vectors built by the active embedder, so an
  index built under a different language degrades to keyword search instead of
  crashing on a vector-dimension mismatch.
• Deep Links now find CJK passages. Keyword Deep Links tokenize the query
  into the same character bigrams the search index uses, so any document
  surfaced by search returns matching passages — previously a 100% search match
  could still report "No matching passages."
• Configured language is honored at every step. docubrowser scan,
  embedding, and database init now resolve the same language as the server. A
  configured ko/ja no longer silently indexes under English, and
  docubrowser ko scan is no longer needed once the language is set.

Upgrade notes

• After upgrading, if Korean/Japanese search or Deep Links behaved oddly under
  v1.5.0, rebuild the index once: stop the server, remove the old database, and
  run docubrowser scan (it now uses your configured language).