fix(backend): rename users table to app_users

Aligns the auth-account table name with the AppUser entity. The historical
mismatch (table 'users' alongside table 'persons') misled schema-first readers
into assuming the two were related; renaming to 'app_users' makes the
deliberate split between auth accounts and historical persons explicit at the
schema layer.

Scope: the table itself, the users_groups join table, and the three FK columns
whose name was literally 'user_id'. Semantic FK columns (audit_log.actor_id,
notifications.recipient_id, document_versions.editor_id, etc.) keep their
names — the role they describe is the documentation, not the type.

Closes #418. Unblocks #407 (REFACTOR-1).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V60__rename_users_to_app_users.sql

@@ -0,0 +1,21 @@
+-- Align the auth-account table name with the AppUser entity (issue #418).
+-- The historical mismatch (table 'users' alongside table 'persons') misleads
+-- schema-first readers into assuming the two are related. Renaming the table to
+-- 'app_users' makes the deliberate split between auth accounts and historical
+-- persons explicit at the schema layer.
+--
+-- Scope: the table itself, the users_groups join table, and the three FK
+-- columns whose name is literally 'user_id'. Semantic FK columns
+-- (audit_log.actor_id, notifications.recipient_id, document_versions.editor_id,
+-- document_comments.author_id, transcription_blocks.created_by/updated_by,
+-- transcription_block_versions.changed_by, document_annotations.created_by,
+-- ocr_training_runs.triggered_by, invite_tokens.created_by, geschichten.author_id)
+-- keep their names — the role they describe is the documentation, not the type.
+
+ALTER TABLE users          RENAME TO app_users;
+
+ALTER TABLE users_groups   RENAME TO app_users_groups;
+ALTER TABLE app_users_groups        RENAME COLUMN user_id TO app_user_id;
+
+ALTER TABLE comment_mentions        RENAME COLUMN user_id TO app_user_id;
+ALTER TABLE password_reset_tokens   RENAME COLUMN user_id TO app_user_id;