feat(backend)!: run on Postgres with a migration-owned schema (#28)

Swap modernc.org/sqlite for jackc/pgx/v5 with no observable change:
same endpoints, same wire format, same updated_at ordering rule.

The schema now comes from numbered SQL embedded in the binary and
applied on startup, one transaction each, recorded in
schema_migrations. That replaces two pieces of SQLite-era machinery,
both deleted rather than ported: the column probing (Postgres has ADD
COLUMN IF NOT EXISTS, and there is no legacy database left to probe)
and the Asura key rewrite, which has run clean on every start for
months now that the userscripts strip build hashes before writing. Its
regexp survives as latest.asuraBuildHash, where the poller still needs
it to scope chapter links to a series whose slug carries a rotating
hash.

Types get real: favorite is a boolean, chapter numbers double
precision, timestamps stay unix-ms bigint. SQLite's null-safe IS NOT
becomes IS DISTINCT FROM, which is what implements the rule that only
reading progress reorders a list. Inside COALESCE/NULLIF the status
and kind parameters need an explicit ::text -- there is no target
column to infer from and Postgres refuses to guess.

Tests lose their free t.TempDir() database, so Docker is now a hard
prerequisite for `go test ./...`: internal/pgtest starts one
postgres:17-alpine per test binary and hands each test a database of
its own.

Also lands CONTEXT.md and the four ADRs written while scoping #18.

BREAKING CHANGE: DB_PATH is retired for DATABASE_URL, which is
required and has no default. Compose gains a postgres service on an
internal network with its own volume; POSTGRES_PASSWORD joins .env.
The old bookmarks-data volume is deliberately left undeclared so
`docker compose down -v` cannot take the pre-migration database with
it. main is not deployable until #25 and #26 land.

Closes #20

Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/latest/sites.go

@@ -5,8 +5,6 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
-
-	"bookmarkmanager/backend/internal/store"
 )
 
 // latestChapter is the newest chapter a series page advertises.
@@ -22,6 +20,12 @@ type latestChapter struct {
 // before using the slug to scope anything.
 var asuraSlugRe = regexp.MustCompile(`/comics/([^/?#]+)`)
 
+// asuraBuildHash matches the trailing "-xxxxxxxx" site-wide build ID Asura
+// appends to every series slug. It rotates on each site redeploy, so it is
+// never part of a stable series_id. Must stay in sync with stripBuildHash in
+// userscript/manga-bookmark.user.js.
+var asuraBuildHash = regexp.MustCompile(`-[0-9a-f]{8}$`)
+
 // demonicChapterRe matches the pre-redirect anchors demonic series pages link
 // through. Both the raw "&" and the HTML-escaped "&" forms occur.
 var demonicChapterRe = regexp.MustCompile(`chaptered\.php\?manga=\d+&(?:amp;)?chapter=([0-9.]+)`)
@@ -74,9 +78,9 @@ func latestChapterFrom(site, seriesURL, body string) (latestChapter, bool) {
 		}
 		// Stored URLs predating a redeploy may carry a stale build hash;
 		// chapter hrefs in the fetched body carry the current one. Strip to
-		// the stable ID (same rule as migrateAsuraKeys) and make the hash
-		// optional in the pattern, so scoping survives rotations.
-		slug := store.AsuraBuildHash.ReplaceAllString(m[1], "")
+		// the stable ID and make the hash optional in the pattern, so scoping
+		// survives rotations.
+		slug := asuraBuildHash.ReplaceAllString(m[1], "")
 		// Compiled per call rather than cached: this runs once per fetch, which
 		// is at most a few times a minute, and the slug varies per series.
 		re = regexp.MustCompile(`/comics/` + regexp.QuoteMeta(slug) + `(?:-[0-9a-f]{8})?/chapter/([0-9.]+)`)