fix: serialize MuPDF metadata reads

[Matbe34]

Summary

PDFService.GetMetadata and GetPageDimensions held `RLock` and called into MuPDF (`s.doc.Metadata()`, `s.doc.Bound()`), but MuPDF's `fz_context` is not safe for concurrent access. Two readers proceeding under `RLock` would race inside MuPDF and crash the test binary mid-run (no `--- FAIL` line, just a package-level abort).

`TestConcurrentOperations` exposes this — fails ~3/5 runs locally on master. The previous green CI runs were luck.

Fix: upgrade those two methods to exclusive `Lock`. `GetPageCount` stays `RLock` (only reads a Go int, no MuPDF re-entry).

Performance impact

Negligible. Both methods are microsecond-scale MuPDF reads; the workload is a single-user desktop app with one PDF open. The "lost parallelism" was undefined behavior that segfaulted under load — not a real perf baseline. `GetPageCount` (the hot path) is unaffected.

Test plan

• `go test -count=1 -run TestConcurrentOperations ./internal/pdf/` — 10/10 PASS after fix (was 3/10 PASS on master)
• Full pdf suite 5/5 PASS after fix