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/migrations/migration_registry.txt

@@ -15,3 +15,4 @@
 20261003110855698-pdf-sources
 20261007205440868-card-buried-at
 20261008165051407-pdf-position
+20261008223835206-dictionaries