feat(i18n): every server A→Z list in the reader's alphabet (E30 S3)

The Postgres image is Alpine, whose en_US.utf8 is byte order: "Életrajzi" and
"Ötödik" sorted after "Zorró" in the feed, the channel lists, the Plex grid,
its genres and collections. `app/collation.py` names the ICU collation of the
reader's language (hu-x-icu puts "Ö" after "O" and "Cs" after "Cu"), wraps
every text ORDER BY with it - the feed's keyset cursor compares with the same
expression, so paging walks the order it sorted in - and orders the lists built
in Python (genre and studio facets, collection members, the discovery re-sort)
through the same collation. Plex title tiebreaks use the shown (HU) title.

backend/app/collation.py

@@ -0,0 +1,39 @@
+"""Alphabetical order in the reader's language (E30 S3).
+
+Why not the database default: the Postgres image is Alpine, and musl's `en_US.utf8` collation is BYTE
+order — every accented initial sorted after "Z" ("Zorró < Életrajzi < Ötödik"), in every A→Z list the
+server orders. Postgres ships the ICU collations on the same image (dev, test, prod alike), so a text
+sort names one: Hungarian puts "Ö" after "O" and "Cs" after "Cu", English folds the accents in.
+
+`collated` wraps an ORDER BY expression — and because a keyset cursor compares with the SAME expression
+(`feedquery.sort_keys`), the cursor pages in the same order it sorted in. `sorted_words` orders a Python
+list the same way through the database, so a list built in Python matches one built in SQL exactly."""
+
+from typing import Any
+
+from sqlalchemy import SQLColumnExpression, column, func, select, values
+from sqlalchemy.orm import Session
+from sqlalchemy.sql.elements import ColumnElement
+from sqlalchemy.types import String
+
+from app.mailtext import normalize
+
+ICU: dict[str, str] = {"en": "en-x-icu", "hu": "hu-x-icu"}
+
+
+def collation(lang: str | None) -> str:
+    """The ICU collation for a UI language; anything unknown sorts as English."""
+    return ICU[normalize(lang)]
+
+
+def collated(expr: SQLColumnExpression[Any], lang: str | None) -> ColumnElement[Any]:
+    """`expr` compared in the reader's alphabet. Case-folded, so "apple" and "Apple" sit together."""
+    return func.lower(expr).collate(collation(lang))
+
+
+def sorted_words(db: Session, words: list[str], lang: str | None) -> list[str]:
+    """`words` in the reader's alphabetical order, by the same ICU collation as `collated`."""
+    if not words:
+        return []
+    w = values(column("w", String), name="words").data([(x,) for x in words])
+    return list(db.scalars(select(w.c.w).order_by(collated(w.c.w, lang), w.c.w)))