feat(server): dictionaries, loaded from files and searched

The server now holds dictionaries for everyone: `dictionary_entry` and
`dictionary_source`, reference data owned by no one, never synced. It
loads the dictionary files in its `dictionaries` folder at every start,
those whose SHA-256 changed: in the background, each source replaced in
one transaction, so searches meanwhile find the old entries; a file it
can't read is logged and left out. All of JMdict loads in under half a
minute. On an instance the folder is deploy/dictionaries, mounted
read-only (compose.yaml); the server's README says how.

DictionaryEndpoint (signed in): the dictionaries with their credits; a
search (exact key, exact loose key, key starting with the query, loose
key starting with it, each in the source's order; katakana as hiragana,
macrons written out), some 150-450 ms over all of JMdict; and the
entries with a label, counted and a page at a time (all JLPT N5 words).

The curator now keeps spellings that aren't quite right apart, as
`looseKeys` (romaji with long vowels typed once): "sushi" finds 寿司
before 数詞 (すうし).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_server/lib/server.dart

@@ -1,4 +1,8 @@
+import 'dart:async';
+import 'dart:io';
+
 import 'package:halkyon_learn_server/src/auth/auth_config.dart';
+import 'package:halkyon_learn_server/src/dictionary/dictionary_loader.dart';
 import 'package:halkyon_learn_server/src/generated/serverpod.dart';
 import 'package:serverpod_auth_idp_server/core.dart';
 
@@ -37,6 +41,17 @@ Future<void> run(List<String> args) async {
       .cron('47 3 * * *')
       .filePruning
       .pruneFiles();
+
+  // The dictionaries the operator put into the server's folder, those that
+  // changed since; in the background, as loading takes a minute or two.
+  unawaited(
+    pod.withSession(
+      (session) => loadDictionaries(
+        session,
+        Directory(Platform.environment['DICTIONARIES_DIR'] ?? 'dictionaries'),
+      ),
+    ),
+  );
 }
 
 /// The identifier of the daily pruning.