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

internal/app now provides the cache, the fetcher, the processor, the
image service, the session manager and the encrypted URL generator, and
handlers.New receives them built. 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, the default
cache_max_bytes included, which internal/app works out before it builds
the cache. 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. The server tests build the pipeline themselves,
as they cannot import internal/app.

Model: opus-5-5

internal/config/cache_max_bytes_internal_test.go

@@ -135,10 +135,38 @@ func TestCacheMaxBytesInvalidValuesAbortStartup(t *testing.T) {
 	}
 }
 
+// TestDefaultCacheMaxBytesAppliesFloor verifies that when 75% of the
+// free space plus what the cache holds is below 500 MiB, the default is
+// floored at DefaultCacheMaxBytesFloor.
+func TestDefaultCacheMaxBytesAppliesFloor(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name      string
+		freeBytes uint64
+	}{
+		{name: "100 MiB free", freeBytes: 104857600},
+		{name: "zero free", freeBytes: 0},
+		{name: "just below floor threshold", freeBytes: 699050665},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := DefaultCacheMaxBytes(tc.freeBytes, 0)
+			if got != DefaultCacheMaxBytesFloor {
+				t.Errorf("DefaultCacheMaxBytes = %d, want floor %d",
+					got, DefaultCacheMaxBytesFloor)
+			}
+		})
+	}
+}
+
 // TestCacheMaxBytesExplicitIsRecorded verifies that an omitted
-// cache_max_bytes is recorded as not explicit, so the cache works out
-// the default when it opens, and that an explicit zero is recorded as
-// explicit, so it disables the disk cache instead.
+// cache_max_bytes is recorded as not explicit, so pixad works out the
+// default when it builds the cache, and that an explicit zero is recorded
+// as explicit, so it disables the disk cache instead.
 func TestCacheMaxBytesExplicitIsRecorded(t *testing.T) {
 	t.Parallel()