feat: dictionaries hold words or characters, and are asked by their source

A dictionary source now says what it holds (`kind`: `words`, the default
for files from before, or `characters`), a string so that an app older
than a new kind leaves it out. Search, countLabelled and labelled take the
dictionary by its source and meaning language, as the unique index names
it, rather than by the language: with KANJIDIC2 beside JMdict, a search
for words would otherwise bring kanji, and JLPT N5 would hold both. The
search reads the language for its query variants from the source's row;
a dictionary the instance doesn't hold finds nothing. The rank index goes
by source too.

Migration: drops and recreates dictionary_entry_rank_idx on (source,
glossLanguage, rank), and adds dictionary_source.kind, text NOT NULL
DEFAULT 'words', so a JMdict loaded before is a dictionary of words. Both
tables are reference data, never synced; the old server keeps working
until the switch (its queries by language just lose the index).

In the app, a studyset's dictionary is one of words unless a kind is
asked for.

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/20261009123850448-dictionary-kind/migration.json

@@ -0,0 +1,78 @@
+{
+  "__className__": "serverpod.DatabaseMigration",
+  "actions": [
+    {
+      "__className__": "serverpod.DatabaseMigrationAction",
+      "type": "alterTable",
+      "alterTable": {
+        "__className__": "serverpod.TableMigration",
+        "name": "dictionary_entry",
+        "schema": "public",
+        "addColumns": [],
+        "deleteColumns": [],
+        "modifyColumns": [],
+        "addIndexes": [
+          {
+            "__className__": "serverpod.IndexDefinition",
+            "indexName": "dictionary_entry_rank_idx",
+            "elements": [
+              {
+                "__className__": "serverpod.IndexElementDefinition",
+                "type": 0,
+                "definition": "source"
+              },
+              {
+                "__className__": "serverpod.IndexElementDefinition",
+                "type": 0,
+                "definition": "glossLanguage"
+              },
+              {
+                "__className__": "serverpod.IndexElementDefinition",
+                "type": 0,
+                "definition": "rank"
+              }
+            ],
+            "type": "btree",
+            "isUnique": false,
+            "isPrimary": false
+          }
+        ],
+        "deleteIndexes": [
+          "dictionary_entry_rank_idx"
+        ],
+        "addForeignKeys": [],
+        "deleteForeignKeys": [],
+        "warnings": []
+      }
+    },
+    {
+      "__className__": "serverpod.DatabaseMigrationAction",
+      "type": "alterTable",
+      "alterTable": {
+        "__className__": "serverpod.TableMigration",
+        "name": "dictionary_source",
+        "schema": "public",
+        "addColumns": [
+          {
+            "__className__": "serverpod.ColumnDefinition",
+            "name": "kind",
+            "fieldName": "kind",
+            "columnType": 0,
+            "isNullable": false,
+            "columnDefault": "'words'",
+            "dartType": "String"
+          }
+        ],
+        "deleteColumns": [],
+        "modifyColumns": [],
+        "addIndexes": [],
+        "deleteIndexes": [],
+        "addForeignKeys": [],
+        "deleteForeignKeys": [],
+        "warnings": []
+      }
+    }
+  ],
+  "warnings": [],
+  "migrationApiVersion": 1
+}