feat: import existing publications from XML (story #05)

scripts/import_publications.py imports all osw=1 publications from
03-data/osws.xml — upserts pub rows, loads cover images, scans
03-data/osdb/{pubId}/ for fruit images and PDFs, links matched fruits
by osdb_number. Idempotent re-runs. 17 unit tests. Makefile updated
to include import_publications_test in make test. Added
scripts/README_import.md with run order. Added spec-first rule to
.claude/CLAUDE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/CLAUDE.md

@@ -0,0 +1,11 @@
+## Taiga
+- TAIGA_URL: https://taiga.db-extern.de
+- TAIGA_PROJECT_SLUG: obstsortendatenbank
+
+## Stories
+Before implementing a story: read its spec file in docs/planning/specs/ first.
+
+## Branching
+Stories developed sequentially; prior story branch may not be merged to main yet.
+Before creating feature branch: run `git branch -r | grep feature/` — if predecessor
+story branch exists on remote and is unmerged, base new branch on it, not main.