Fix German grammatical cases in naturaltime

[Boulea7]

Fixes #263

Changes proposed in this pull request:

• Use German dative forms in naturaltime, such as vor einer Stunde and in 2 Tagen, with ordinary-translation fallback and unchanged public signatures.
• Detect now from the duration rather than translated text, fixing Turkish one-second past and future output. Custom catalogs that relied on matching non-moment text to a moment will now produce relative time.
• Add real-catalog regressions and verify extraction, merge, and compilation round trips.

Validation (Python 3.11, Linux): 986 tests passed; 22 prek hooks passed. Full prek was blocked installing yamlfmt by a proxy.golang.org timeout on both main and this branch.

Local measurements: French missing-context fallback adds about 0.45 µs per naturaltime call (+10.8%).