fix: permanent owner password persistence with SeedAudit guard

Root cause: Dual-source architecture for owner password (Gitea secret
ENV_OWNER_PASSWORD vs host .env OWNER_PASSWORD) caused drift when
the DB was ever re-seeded or the volume recreated.

Changes:
- Add SeedAudit entity + migration to track one-time seed operations
- EnsureDatabaseAsync checks SeedAudit BEFORE seeding — owner is never
  re-created even if the Users table is wiped
- Deploy and rollback workflows now read OWNER_PASSWORD from the host's
  persistent .env (single source of truth) instead of Gitea secrets
- compose.yaml documented: OWNER_PASSWORD only used during initial seed
- Cleanup: .gitignore extended for core dumps, changelog/deployment.md
  updated with 2026-06-20 session notes

After this fix the DB is the single source of truth for the owner
password after initial seed. The host .env is the single reference
for the initial value.

backend/Extensions/ApplicationBuilderExtensions.cs

@@ -13,6 +13,8 @@ public static class ApplicationBuilderExtensions
 {
     /// <summary>
     /// Applies pending EF Core migrations and seeds the initial owner account if none exist.
+    /// Uses a <see cref="SeedAudit"/> guard so the owner is never re-created even if all users
+    /// are deleted — the DB is the single source of truth for the owner password after first seed.
     /// </summary>
     public static async Task EnsureDatabaseAsync(this WebApplication app)
     {
@@ -23,6 +25,11 @@ public static class ApplicationBuilderExtensions
             var db = scope.ServiceProvider.GetRequiredService<NexusDbContext>();
             await db.Database.MigrateAsync();
 
+            const string seedKey = "owner_created";
+            var alreadySeeded = await db.SeedAudits.AnyAsync(s => s.Key == seedKey);
+            if (alreadySeeded)
+                return;
+
             var ownerEmail = configuration["Owner:Email"]?.Trim().ToLowerInvariant();
             var ownerPassword = configuration["Owner:Password"];
             var ownerDisplayName = configuration["Owner:DisplayName"]?.Trim();
@@ -58,6 +65,11 @@ public static class ApplicationBuilderExtensions
                     Console.Error.WriteLine($"[nexus] Initial owner credentials generated: displayName={initialDisplayName}, password={initialPassword}");
                 }
             }
+
+            // Record the seed attempt regardless of whether users already existed.
+            // This prevents re-seeding even if the Users table is wiped.
+            db.SeedAudits.Add(new SeedAudit { Key = seedKey });
+            await db.SaveChangesAsync();
         }
     }