Fix import FK fault: derived/back-linked litter referenced a not-yet-inserted parent

BUG (blocked the supervised live re-import): /import/execute threw
Npgsql 23503 FK_Litters_Gerbils_FatherId and rolled back. Root cause was
INSERT ORDERING — synthesized derived litters were SaveChanges()'d BEFORE
the parent gerbils (created later in the animal loop), so the litter's
Father/MotherId pointed at rows that didn't exist yet. The dry-run and the
EF in-memory test provider don't enforce FKs, so it slipped through.

Fix:
- Stage synthesized litters in the context but DON'T save them early; the
  single SaveChanges after the animal loop lets EF order parents → litters →
  offspring (all FKs nullable). Saving litters first was the fault.
- FK-integrity guard (god's spec): compute the persisted set (existing DB +
  this run's loadable) and null out any litter parent FK not in it; SKIP a
  derived litter whose BOTH parents are unresolvable (offspring loads with
  LitterId=null). Quarantined parents already resolve to null via
  ResolveParentGid; this is defense-in-depth + makes the invariant explicit.
- Report litterParentFksDropped + derivedLittersSkipped (LitterSummary) +
  a German note — so a green dry-run (0/0) GUARANTEES execute won't FK-fault.

Tests: two SQLite-backed regressions (SQLite enforces FKs, unlike the
in-memory provider) — a derived litter with NEW chart parents executes
without throwing, and a quarantined parent leaves that FK null. 120 C#
tests + python green; no schema change (has-pending clean).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

GerbilManagerWebAPI/Import/ImportModels.cs

@@ -81,7 +81,8 @@ namespace GerbilManagerWebAPI.Import
     /// Zuchtname animals made resident because they parented a Clan offspring (rule b).</summary>
     public sealed record ResidencySummary(int Resident, int External, int FlippedByParentRule);
 
-    public sealed record LitterSummary(int InSource, int Created, int AlreadyImported, int DerivedFromChart = 0);
+    public sealed record LitterSummary(int InSource, int Created, int AlreadyImported,
+        int DerivedFromChart = 0, int DerivedSkipped = 0, int ParentFksDropped = 0);
 
     public sealed record AnimalSummary(
         int InSource,