refactor(backend): replace coravel with hangfire for background jobs

for *some reason*, coravel locks a persistent job queue behind a
paywall. this means that if the server ever crashes, all pending jobs
are lost. this is... not good, so we're switching to hangfire for that
instead.

coravel is still used for emails, though.

BREAKING CHANGE: Foxnouns.NET now requires Redis to work. the EFCore
storage for hangfire doesn't work well enough, unfortunately.

Foxnouns.Backend/Jobs/CreateFlagInvocable.cs

@@ -12,49 +12,53 @@
 //
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
-using Coravel.Invocable;
 using Foxnouns.Backend.Database;
 using Foxnouns.Backend.Database.Models;
 using Foxnouns.Backend.Extensions;
 using Foxnouns.Backend.Services;
+using Hangfire;
 using Microsoft.EntityFrameworkCore;
 
 namespace Foxnouns.Backend.Jobs;
 
-public class CreateFlagInvocable(
+public class CreateFlagJob(
     DatabaseContext db,
     ObjectStorageService objectStorageService,
     ILogger logger
-) : IInvocable, IInvocableWithPayload<CreateFlagPayload>
+)
 {
-    private readonly ILogger _logger = logger.ForContext<CreateFlagInvocable>();
-    public required CreateFlagPayload Payload { get; set; }
+    private readonly ILogger _logger = logger.ForContext<CreateFlagJob>();
 
-    public async Task Invoke()
+    public static void Enqueue(CreateFlagPayload payload)
+    {
+        BackgroundJob.Enqueue<CreateFlagJob>(j => j.InvokeAsync(payload));
+    }
+
+    public async Task InvokeAsync(CreateFlagPayload payload)
     {
         _logger.Information(
             "Creating flag {FlagId} for user {UserId} with image data length {DataLength}",
-            Payload.Id,
-            Payload.UserId,
-            Payload.ImageData.Length
+            payload.Id,
+            payload.UserId,
+            payload.ImageData.Length
         );
 
         try
         {
             PrideFlag? flag = await db.PrideFlags.FirstOrDefaultAsync(f =>
-                f.Id == Payload.Id && f.UserId == Payload.UserId
+                f.Id == payload.Id && f.UserId == payload.UserId
             );
             if (flag == null)
             {
                 _logger.Warning(
                     "Got a flag create job for {FlagId} but it doesn't exist, aborting",
-                    Payload.Id
+                    payload.Id
                 );
                 return;
             }
 
             (string? hash, Stream? image) = await ImageObjectExtensions.ConvertBase64UriToImage(
-                Payload.ImageData,
+                payload.ImageData,
                 256,
                 false
             );
@@ -68,7 +72,7 @@ public class CreateFlagInvocable(
         }
         catch (ArgumentException ae)
         {
-            _logger.Warning("Invalid data URI for flag {FlagId}: {Reason}", Payload.Id, ae.Message);
+            _logger.Warning("Invalid data URI for flag {FlagId}: {Reason}", payload.Id, ae.Message);
         }
 
         throw new NotImplementedException();