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/Dockerfile

@@ -2,6 +2,7 @@
 
 # --- build stage: compile a static, CGO-free binary ---
 FROM golang:1.26-alpine AS build
+ARG COVER_DIR=/covers
 WORKDIR /src
 
 # Dependencies first for layer caching (changes rarely).
@@ -19,11 +20,15 @@ COPY internal/ ./internal/
 # -trimpath + -ldflags strip paths and debug info for a smaller image.
 RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w" -o /out/server .
 
+# Create the source directory; runtime COPY sets ownership for the named volume.
+RUN mkdir -p "$COVER_DIR"
+
 # --- runtime stage: distroless static, non-root ---
 FROM gcr.io/distroless/static:nonroot
+ARG COVER_DIR=/covers
 WORKDIR /
+COPY --from=build --chown=65532:65532 ${COVER_DIR} ${COVER_DIR}
 COPY --from=build /out/server /server
-
 EXPOSE 8080
 USER nonroot:nonroot
 ENV PORT=8080