refactor(db): migrate all ids to a uniform canonical 128-bit base62 encoding (#5824)

* refactor(model): extract canonical 128-bit base62 id codec

* feat(model): generate random ids as canonical 128-bit base62 values

* feat(scanner): emit legacy PIDs in canonical base62 encoding

* feat(db): add id canonicalization transform for the uniform-ids migration

* feat(db): migrate all ids to canonical 128-bit base62 encoding

* fix(db): canonicalize ids in junction tables and JSON columns

* chore(jellyfin): update id-family notes for uniform canonical ids

* test(ids): harden codec input contract and migration edge coverage

* refactor(model): use log.Fatal for Encode128 contract guard per project convention

* fix(db): force full rescan after id migration for legacy PID configs

* test(db): guard id-column inventory against schema drift

* refactor(ids): compile-time Encode128 contract and unified column rewrite helper

* refactor(db): apply review feedback to id migration

Filter empty strings in collectColumn's SQL, reuse a prepared statement
for rewriteColumn updates, and clarify the legacy ID functions' comment
now that they emit the canonical encoding.

* feat(auth): split session and public-link JWT secrets, rotating sessions on id migration

* test(subsonic): initialize public token secret in helpers suite

The suite sets auth.TokenAuth directly instead of calling auth.Init, so the
new PublicTokenAuth was nil whenever Ginkgo's spec order ran a helpers spec
before any spec that calls auth.Init, panicking in publicurl.ImageURL.

* refactor(db): inline canonicalID into its only consumer, the uniform-ids migration

* refactor(model): rename Encode128/Decode128 to Encode/Decode

With every id now exactly 128 bits, the width suffix is redundant; the
package-qualified id.Encode/id.Decode carries the same information.

* test(db): make the id-columns guard classify JSON columns too

The guard only inspected columns named id/pid/*_id, so it could not see ids
embedded in JSON. Widen it to *_ids and to every JSON column, and drive the
"covered" set from a new embeddedIDColumns list instead of the inline calls
in the migration.

Every JSON column the schema has now carries a verdict. The four denormalized
caches -- media_file/album.participants, media_file/album.tags,
album.folder_ids and artist.similar_artists -- hold only artist, tag and
folder ids. Those all come from id.NewHash, whose 22-char base62 encoding of
a 128-bit MD5 is already in canonical range, so canonicalID is the identity
on them and the migration correctly leaves them alone. A new codec test pins
that invariant, since the exemptions depend on it.

Verified on a copy of a 727MB/96k-track production database: canonicalizing
those four columns changed zero rows, and artist, tag and folder ids were
themselves unchanged by the migration (only media_file ids moved, 95108 of
96666).

plugins/host_artwork_test.go

@@ -225,7 +225,7 @@ func decodeArtworkURL(artworkURL string) model.ArtworkID {
 	}
 
 	// Decode JWT token
-	token, err := auth.TokenAuth.Decode(tokenPart)
+	token, err := auth.PublicTokenAuth.Decode(tokenPart)
 	Expect(err).ToNot(HaveOccurred(), "Failed to decode JWT token")
 
 	c := auth.ClaimsFromToken(token)