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>

docker-compose.yml

@@ -11,7 +11,10 @@
 
 services:
   bookmark-api:
-    build: ./backend
+    build:
+      context: ./backend
+      args:
+        COVER_DIR: ${COVER_DIR:?set COVER_DIR in .env}
     image: bookmarkmanager-backend:latest
     container_name: bookmark-api
     restart: unless-stopped
@@ -26,6 +29,9 @@ services:
       # The bookmarks database. Host is the compose service name; the password
       # comes from .env so it is never committed.
       DATABASE_URL: ${DATABASE_URL:-postgres://bookmarks:${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env}@postgres:5432/bookmarks?sslmode=disable}
+      # Required path inside the API. The build seeds ownership at this path
+      # and the named volume below mounts there.
+      COVER_DIR: ${COVER_DIR:?set COVER_DIR in .env}
       PORT: "8080"
       # Log timestamps only. Go's `log` stamps lines in local time, and this
       # service has no other use for a zone: bookmark timestamps are unix ms
@@ -80,6 +86,8 @@ services:
       # no rebuild, no restart. `git pull` restores the committed version, which
       # is why a redeploy always ships the repo's script.
       - ./userscript:/userscript:ro
+      # Content-addressed cover bytes survive API restarts and redeploys.
+      - cover-data:${COVER_DIR:?set COVER_DIR in .env}
     # Bound to loopback only: the proxy (or curl during smoke test) reaches it,
     # the public internet does not.
     ports:
@@ -112,6 +120,7 @@ services:
 
 volumes:
   postgres-data:
+  cover-data:
   # The pre-Postgres SQLite volume (bookmarks-data) is deliberately no longer
   # declared here: undeclared means `docker compose down -v` cannot take it
   # with the rest, so the old database survives the cutover until someone