Build the image pipeline in pixad's fx graph (closes #313)

The new package internal/providers builds the cache, the fetcher, the
processor, the image service, the session manager and the encrypted URL
generator from the config. pixad's fx graph provides them with
providers.Constructors(), and handlers.New receives them built. The
server tests build the same graph, with only the fetcher's dial function
replaced. database.New opens the database, so these parts can read it
while fx builds them; the cache's stop hook still runs before the
database's.

Each setting's default is declared in internal/config, and
internal/providers works out the default cache_max_bytes. The cache's
MaxBytes is the one value for the disk cache limit, and 0 turns the disk
cache off. The fetcher and the processor take explicit values.

Model: opus-5-5

internal/imgcache/concurrent_misses_internal_test.go

@@ -455,7 +455,7 @@ func TestService_Get_WaiterGetsProcessingTimedOut(t *testing.T) {
 
 	svc, fixtures, fetcher := setupHeldFetchService(t)
 	svc.processor = imageprocessor.New(
-		imageprocessor.Params{MaxConcurrentProcessing: 1},
+		imageprocessor.Params{MaxInputBytes: 50 << 20, MaxConcurrentProcessing: 1},
 	)
 
 	holdProcessingSlot(t, svc.processor)