Fix flexible-wish double-booking, extend calendar cache to 2055, add journey tests

Second real bug found by driving the running app while the suites were green.

Flexible wishes were funded twice across propose -> apply -> re-propose. Because
planned_leave_day carries no wish_id, the engine could not tell that applied days
were that wish's own prior occurrence; it treated them as occupied space, routed
the candidate search around them, and funded the year again on shifted dates while
reporting weeks_funded 1.0. Applying twice turned "one week each spring" into 18
paid days instead of 9. Fixed wishes were unaffected and stay unaffected.

Fix (allocation_service.py), per the approved approach of counting already-applied
days inside the bounding range rather than adding a wish_id column:
- _Ledger._persisted_lookup snapshots existing days BEFORE this run folds in its
  own freshly-computed windows, so a fixed wish's new placement is never mistaken
  for a flexible wish's history
- _reuse_already_funded_window finds this wish's persisted dates within its bound
  and reuses the window containing them, via the ordinary fund_window path
- full funding therefore yields zero new days and partial funding tops up exactly
  the remainder, with no duplication and no false shortfall
- with no persisted dates, or if the reused window is no longer collision-free, it
  falls through to the original search, so an unsatisfied year is never frozen

Calendar cache extended 2026-2035 -> 2026-2055 (39KB -> 116KB) and the allocator
switched to strict, so an uncached year raises UncachedYearError -> ValidationError
-> HTTP 422 instead of silently treating Julafton, Nyarsafton and Midsommarafton as
working days. Also fixed normalize_day dropping the second name when two holidays
share a date (Nationaldagen was being lost). 2049 and 2055 legitimately have 17 red
days, not 18: Pingstdagen coincides with Nationaldagen.

New backend/tests/integration/test_user_journey.py drives the real app over HTTP
across seven journeys: the double-booking guard, the fixed-wish control, apply
idempotency, fixed-never-displaced-by-flexible, flexible wishes tracking Easter
per year, age-4 arithmetic self-consistency, and a recursive check that no Swedish
text reaches the wire now that everything is message_key + data.

New backend/scripts/seed_demo.py seeds a realistic workspace through the real
service layer for manual testing: demo@example.com / DemoPass123!

Backend 414 -> 435 tests. Frontend unchanged at 705.

Still not verified in a browser.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RXMhc9nT6sNz1SL4j8dznL

backend/README.md

@@ -29,6 +29,24 @@ app/
   utils/               # errors, logger
 ```
 
+## Demo data (manual/browser testing)
+
+`scripts/seed_demo.py` seeds a realistic, ready-to-explore workspace through the
+normal service/repository layers (no raw SQL) — a couple workspace with a
+partner, a child born ~6 months ago, vacation rows for only some years (so the
+`is_assumed` badge is visible), day overrides, and a spread of leave wishes
+(fixed, flexible, and one deliberately underfunded to show a shortfall +
+remedies + surplus advice). Login credentials are fixed and printed at the end.
+
+```bash
+uv run python scripts/seed_demo.py            # create once; re-running is a no-op
+uv run python scripts/seed_demo.py --reset     # wipe the demo workspace and rebuild
+```
+
+Then open `http://localhost:5173` and log in with the printed `demo@example.com`
+credentials. Safe to re-run: without `--reset` it detects the existing demo
+user and changes nothing; `--reset` only ever deletes the one demo workspace.
+
 ## Reference vertical slice
 
 `VacationDays` is implemented end-to-end (entity → schema → repo interface →