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>

src/lib/processors/plex-recently-added.processor.ts

@@ -106,7 +106,7 @@ export async function processPlexRecentlyAddedCheck(payload: PlexRecentlyAddedPa
             author: item.author || 'Unknown Author',
             narrator: item.narrator,
             summary: item.description,
-            duration: item.duration ? item.duration * 1000 : null, // Convert seconds to milliseconds
+            duration: item.duration ? BigInt(Math.round(item.duration * 1000)) : null, // Convert seconds to milliseconds
             year: item.year,
             asin: item.asin,  // Store ASIN from library backend
             isbn: item.isbn,  // Store ISBN from library backend
@@ -146,7 +146,7 @@ export async function processPlexRecentlyAddedCheck(payload: PlexRecentlyAddedPa
             author: item.author || existing.author,
             narrator: item.narrator || existing.narrator,
             summary: item.description || existing.summary,
-            duration: item.duration ? item.duration * 1000 : existing.duration,
+            duration: item.duration ? BigInt(Math.round(item.duration * 1000)) : existing.duration,
             year: item.year || existing.year,
             asin: item.asin || existing.asin,  // Update ASIN if available
             isbn: item.isbn || existing.isbn,  // Update ISBN if available

src/lib/processors/scan-plex.processor.ts

@@ -90,7 +90,7 @@ export async function processScanPlex(payload: ScanPlexPayload): Promise<any> {
               author: item.author || existing.author,
               narrator: item.narrator || existing.narrator,
               summary: item.description || existing.summary,
-              duration: item.duration ? item.duration * 1000 : existing.duration, // Convert seconds to milliseconds
+              duration: item.duration ? BigInt(Math.round(item.duration * 1000)) : existing.duration, // Convert seconds to milliseconds
               year: item.year || existing.year,
               asin: item.asin || existing.asin,  // Store ASIN from library backend
               isbn: item.isbn || existing.isbn,  // Store ISBN from library backend
@@ -132,7 +132,7 @@ export async function processScanPlex(payload: ScanPlexPayload): Promise<any> {
               author: item.author || 'Unknown Author',
               narrator: item.narrator,
               summary: item.description,
-              duration: item.duration ? item.duration * 1000 : null, // Convert seconds to milliseconds
+              duration: item.duration ? BigInt(Math.round(item.duration * 1000)) : null, // Convert seconds to milliseconds
               year: item.year,
               asin: item.asin,  // Store ASIN from library backend (Plex or Audiobookshelf)
               isbn: item.isbn,  // Store ISBN from library backend