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/internal/store/migrations/0008_filesystem_covers.sql

@@ -0,0 +1,9 @@
+-- Cover bytes move out of Postgres. Existing rows are intentionally dropped:
+-- the old kagane path already refetches missing Covers on demand.
+DROP TABLE covers;
+
+CREATE TABLE covers (
+  address      text PRIMARY KEY,
+  path         text NOT NULL,
+  content_type text NOT NULL
+);