Postgres perf, path migration, and shutdown race fixes

- Major query performance boost: replaced correlated subqueries with DistinctBy() in BaseItemRepository, added episode deduplication index, and increased default command timeout to 120s.
- Fixed LibraryMonitor shutdown race: added disposal checks and exception handling in FileRefresher to prevent ObjectDisposedException.
- Added PowerShell and SQL utilities for fixing Linux paths in config files and database; new scripts for WebDir and path migration.
- Auto-fix for WebDir in startup.json on load; new helper scripts and docs.
- Added automated weekly DB performance monitoring scripts and reporting.
- Expanded documentation and README for all fixes, scripts, and migration steps.
- Updated SQL scripts for index creation and diagnostics; improved output handling.
- Updated db-config defaults and added new diagnostic/report files.

sql/add_presentationuniquekey_index.sql

@@ -0,0 +1,23 @@
+-- PostgreSQL Performance Fix for Episode Queries
+-- Run this script to add the missing index for PresentationUniqueKey
+-- This will significantly speed up episode deduplication queries
+\pset pager off
+
+-- Create the index (CONCURRENTLY means it won't lock the table during creation)
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_baseitems_presentationuniquekey_episodes
+ON library."BaseItems" ("PresentationUniqueKey", "TopParentId", "IsVirtualItem")
+WHERE "Type" = 'MediaBrowser.Controller.Entities.TV.Episode' AND "PresentationUniqueKey" IS NOT NULL;
+
+-- Verify the index was created
+SELECT 
+    schemaname,
+    tablename,
+    indexname,
+    indexdef
+FROM pg_indexes
+WHERE schemaname = 'library' 
+  AND indexname = 'idx_baseitems_presentationuniquekey_episodes';
+
+-- Check index size
+SELECT 
+    pg_size_pretty(pg_relation_size('library.idx_baseitems_presentationuniquekey_episodes'::regclass)) AS index_size;