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>

REDEPLOY.md

@@ -62,6 +62,10 @@ $COMPOSE exec -T postgres psql -U bookmarks -d bookmarks -c '\dt'
 # -> bookmarks, covers, readers, schema_migrations, series, sessions
 ```
 
+The `covers` table is metadata only after the filesystem cutover: bytes live in
+the separate `cover-data` volume. Back that volume up with the database dump;
+restoring only Postgres leaves stored Cover addresses without files.
+
 Inside the container that connects over the local socket as the `bookmarks`
 superuser, so no password is needed anywhere in this section. `-T` is not
 optional: without it Compose allocates a TTY, which rewrites `\n` to `\r\n` and
@@ -455,7 +459,7 @@ and skipped, stored covers still served.
 | Dump is 0 bytes, or `pg_restore`: `did not find magic string in file header` | You ran `exec` without `-T`. The allocated TTY rewrites newlines in the binary stream and corrupts the archive in flight (§1). |
 | `git pull`: `could not read Username for 'https://…'` | The checkout's remote is the HTTPS clone URL and the server has no credential helper, so the pull prompts into a closed stdin. Switch it to SSH once — `git remote set-url origin ssh://git@gitea.violetcrown.my.id:2222/sulthan/mangaBookmark.git`. Gitea's SSH listens on **2222**, not 22; port 22 is the host's own sshd and answers `Permission denied (publickey)` no matter which key is registered. |
 | kagane rows stopped updating after a redeploy | Check `BROWSER_WS_URL` survived the `.env` edit and still names the home machine's tailnet **IP**. A hostname 500s at `/json/version`; an empty value disables the browser silently. Plain-TLS sites keep working either way, which is why this is easy to miss. |
-| kagane covers went blank in the web UI | They should not — covers are rows in `covers`, not an in-process cache. `$COMPOSE exec -T postgres psql -U bookmarks -d bookmarks -c 'select count(*) from covers'`. Zero after a restore means the dump predates the covers table; they refill on the next poll of each series. |
+| kagane covers went blank in the web UI | Covers use the `cover-data` volume now. Restore/check that volume alongside Postgres; rows in `covers` are metadata only. If the database has rows but files are missing, the next browser-backed request refetches them; without a browser it remains a 404. |
 | Browser unit will not start: `set BROWSER_BIND_ADDR to this machine's tailnet IP` | `chrome/.env` is missing or the variable is empty. It has no default on purpose — an unset value must fail the deploy rather than publish an unauthenticated CDP port to the LAN. |
 | `bookmark-browser` shows `OOMKilled true` | The cap did its job. Read `docker logs bookmark-browser` before raising it — the sizing and what the cap protects are in ADR-0006. |