fix: use BigInt for PlexLibrary.duration to prevent INT4 overflow

The duration column (Int/int4, max ~2.15B) overflows when storing
millisecond values for items with large durations from Audiobookshelf
or Plex backends. Change to BigInt (int8) and wrap duration calculations
in BigInt() at the Prisma write boundary.

Changes:
- prisma/schema.prisma: PlexLibrary.duration Int? → BigInt?
- plex-recently-added.processor.ts: BigInt(Math.round(...)) wrapping
- scan-plex.processor.ts: same BigInt wrapping
- documentation/backend/database.md: updated duration type notation

Fixes #193

Co-Authored-By: Oz <oz-agent@warp.dev>

prisma/schema.prisma

@@ -132,7 +132,7 @@ model PlexLibrary {
   author     String
   narrator   String?
   summary    String?  @db.Text
-  duration   Int? // Duration in milliseconds (Plex format)
+  duration   BigInt? // Duration in milliseconds (Plex format)
   year       Int?
   userRating Decimal? @map("user_rating") @db.Decimal(3, 1) // User's rating (0-10 scale from Plex)