Spec #135: owner data-correction actions — Latest Chapter, series_url, Cover, orphan removal (#156)

Implements spec #135 (spec 2 of 4, derived from wayfinder map #114; decisions settled in #120/#121/#125/#131). Blocked-by #134 is merged, so this lands on `main`.

Four owner actions the dashboard can now perform, one ticket each:

- **#149** — Latest Chapter correction: one numeric input, overwritten by the next machine write.
- **#151** — Series URL repair: owner-typed, gated by the poller's own fetch gate.
- **#150 / #153 / #154** — Cover replacement: addresses derived from bytes (`#150`), a Forced Poll replaces the Cover while an ordinary pass still only fills a blank one (`#153`), and byte reclamation is one guarded helper, file first / covers row last (`#154`).
- **#155** — Orphan removal: one Series at a time, with the foreign key as the guard.

Plus **#152** — Latest Chapter provenance: one derived line naming the actor class, so an owner can tell a hand-edited number from a machine read.

- Migration `0015_latest_correction.sql` adds the correction/provenance columns; `0009` now derives cover addresses from bytes.
- ADR `0014-cover-addresses-from-bytes.md` records the address scheme.

Backend tests cover the store, poller, admin handlers, and web routes (`go test ./...`, needs Docker).

Reviewed-on: #156
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/store/migrations/0009_series_cover_address.sql

@@ -1,8 +1,15 @@
 -- The Cover splits into two facts. `cover` keeps the third-party address the
--- bytes come from, which is what the acquisition path refetches and dedupes
--- on; `cover_address` is the content address of the bytes once they are
--- actually stored, and is what the wire's absolute URL is built from.
+-- bytes come from, which is what the refetch path dedupes on; `cover_address`
+-- is the content address of the bytes once they are actually stored, and is
+-- what the wire's absolute URL is built from.
 --
 -- Empty `cover_address` therefore means "no Cover yet" rather than "a Cover
 -- that 404s", which is the distinction the API and the UI both depend on.
+--
+-- The content address was originally the hex SHA-256 of the source URL
+-- (ADR-0007). Since ADR-0014 it is the hex SHA-256 of the bytes themselves,
+-- so a re-art behind the same URL is a new address. Rows written before
+-- ADR-0014 keep their URL-derived addresses; they are never rehashed and heal
+-- into byte addressing on their first forced replacement. Both derivations
+-- share the 64-hex-digit shape, so the serving guard is unchanged.
 ALTER TABLE series ADD COLUMN cover_address text NOT NULL DEFAULT '';

backend/internal/store/migrations/0015_latest_correction.sql

@@ -0,0 +1,4 @@
+-- latest_corrected_at is the "the current Latest Chapter is the owner's" stamp
+-- (#149). Written by the Correction; zeroed by every machine write of the
+-- value. Zero means never corrected.
+ALTER TABLE series ADD COLUMN latest_corrected_at bigint NOT NULL DEFAULT 0;