Move cover bytes to content-addressed filesystem storage (#65)

Refs #56

## Summary

Moves Kagane cover bytes out of Postgres bytea storage into an immutable, content-addressed filesystem store. Reader-visible behavior remains unchanged: the existing session-gated route serves stored bytes, missing bytes use the existing browser fetch path, and no browser still returns a missing cover.

## Changes

- Added migration 0008, which drops the legacy `covers` table and recreates it with only `address`, `path`, and `content_type`. Existing byte rows are intentionally dropped.
- Added SHA-256 source-URL addressing with two-level sharding (`ab/cd/<sha256>`). Writes use a temp file plus atomic link; reads validate the stored relative path before opening it.
- Made `COVER_DIR` required in runtime config and Compose. Compose passes it as a Docker build argument and volume target, so custom durable paths keep image ownership, runtime config, and the named `cover-data` volume aligned.
- Updated every `store.Open` caller and documented configuration, deployment, backup, and troubleshooting behavior.
- Added filesystem, restart, migration-drop, no-browser, and content-addressing coverage.

## Verification

- `go test ./...`
- `CGO_ENABLED=0 go build ./...`
- `docker build --build-arg COVER_DIR=/data/covers -t manga-bookmark-cover-check-custom ./backend`
- `docker compose config --format json` confirms custom `COVER_DIR` is the volume target
- `git diff --check origin/main`
- LSP diagnostics clean for touched Go files

Parents #47 and #55 remain open as required by #56.

Reviewed-on: #65
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/cover_test.go

@@ -101,11 +101,12 @@ func TestKaganeCoverServesStoredBytesWithoutBrowser(t *testing.T) {
 
 func TestKaganeCoverServesPersistedBytesAfterRestart(t *testing.T) {
 	url := pgtest.URL(t)
+	coverDir := t.TempDir()
 	owner := store.Owner{
 		DiscordID: "cover-owner",
 		TokenHash: sha256.Sum256([]byte("cover-owner-token")),
 	}
-	first, err := store.Open(url, owner)
+	first, err := store.Open(url, owner, coverDir)
 	if err != nil {
 		t.Fatalf("Open: %v", err)
 	}
@@ -117,7 +118,7 @@ func TestKaganeCoverServesPersistedBytesAfterRestart(t *testing.T) {
 		t.Fatalf("close first store: %v", err)
 	}
 
-	second, err := store.Open(url, owner)
+	second, err := store.Open(url, owner, coverDir)
 	if err != nil {
 		t.Fatalf("reopen: %v", err)
 	}