feat: normalize subscription architecture, integrate pgvector, and implement Stripe webhook subscription management.

src/NexusReader.Infrastructure/Services/EpubService.cs

@@ -41,7 +41,20 @@ public class EpubService : IEpubService
                 return Result.Fail($"EPUB file not found. Checked {searchPaths.Count} locations, including: {string.Join(", ", searchPaths.Take(3))}");
             }
 
-            EpubBook book = await EpubReader.ReadBookAsync(fullPath);
+            if (!File.Exists(fullPath))
+            {
+                return Result.Fail($"EPUB file at '{fullPath}' is not accessible or does not exist.");
+            }
+
+            EpubBook book;
+            try 
+            {
+                book = await EpubReader.ReadBookAsync(fullPath);
+            }
+            catch (Exception ex)
+            {
+                return Result.Fail(new Error($"Failed to parse EPUB file. It might be corrupted or in use. Path: {fullPath}").CausedBy(ex));
+            }
             var blocks = new List<ContentBlock>();
             int totalWordCount = 0;
             int blockCounter = 0;