fix(PROJ-35): hashMailID liefert int64 statt uint64

Manticore akzeptiert in `id`-bigint nur signed int64. Der mysql-Treiber
serialisiert Parameter als Dezimal-String → uint64-Werte > int64.MaxValue
führten zu "number ... is out of range". Fix: int64(h.Sum64())
verlustfreier Bit-Cast — bestehende Dokumente bleiben erreichbar.

Auch: PROJ-35-Spec auf In Progress + Implementation Notes/Pitfalls/QA-Block,
INDEX.md-Status-Update.

internal/index/manticore.go

@@ -368,11 +368,20 @@ func (idx *manticoreIndex) Close() error {
 
 // ── helpers ────────────────────────────────────────────────────────────────
 
-// hashMailID returns a stable uint64 row ID derived from the mail's SHA-256 string ID.
-func hashMailID(id string) uint64 {
+// hashMailID returns a stable signed int64 row ID derived from the mail's
+// SHA-256 string ID. Manticore's `id` column is bigint (signed int64); the
+// mysql driver formats parameters as decimal strings, and Manticore rejects
+// values outside the signed range with "number ... is out of range".
+//
+// We FNV-64a-hash the string and reinterpret the resulting uint64 as int64
+// (verlustfreier Bit-Cast). All bit patterns map 1:1, so already-indexed
+// documents stay reachable — values whose top bit was set previously
+// became negative IDs; their bit pattern is unchanged, only the SQL
+// rendering differs and now matches what Manticore expects.
+func hashMailID(id string) int64 {
 	h := fnv.New64a()
 	h.Write([]byte(id))
-	return h.Sum64()
+	return int64(h.Sum64())
 }
 
 // manticoreTableName returns the RT table name for a given tenant.