Prod: add vacancy city metadata API

[TThanos3000]

Summary

Production port of the vacancy city metadata functionality verified on dev in PR #706 (feature commit 446cab5820ff32e0b74d7ec82a5f4fbf28df5d2d). The port is applied semantically on top of current master base 1aee63c8a506404282170909152da3756bd80255; dev was not merged and its merge/deploy fixes were not cherry-picked.

Changes

• adds nullable Vacancy.city (max_length=255) through migration vacancy.0010_vacancy_city;
• requires a trimmed city for office and hybrid vacancies;
• clears city for remote vacancies;
• validates the resulting object state for create, PUT, and PATCH;
• accepts legacy Angular work format смешанная and stores/returns canonical смешанный формат;
• exposes city in create, detail, list, catalog, legacy project nested, and workspace vacancy contracts;
• adds editable metadata to the existing project nested vacancy contract;
• adds regression coverage for create/update/normalization/legacy compatibility and production catalog/workspace contracts.

Production-only behavior preserved

• current VacancyCatalogSerializer and public catalog contract;
• existing response serializers, response-count annotations and selectors;
• current vacancy permissions, including staff/superuser management access;
• all newer response lifecycle and query optimizations from master.

No production file was replaced wholesale with its dev version.

Migration impact

vacancy/migrations/0010_vacancy_city.py follows 0009_vacancy_specialization and only adds a nullable column. Existing production vacancies remain valid with city = NULL; no data backfill from Project.region is performed.

Checks

• GitHub CI repository-wide Flake8: passed (13s)
• GitHub CI full backend suite: passed, 1225/1225 (22m24s)
• PR Backend PostgreSQL CI: passed, including connection/locking verification, Django configuration/models, migration consistency/application, targeted locking/constraint tests, and full suite 1225/1225 (7m29s)
• Black check for all changed Python files: passed
• Flake8 for all changed Python files: passed
• python manage.py check --tag models: passed
• python manage.py makemigrations --check --dry-run --skip-checks: passed (No changes detected)
• migration graph assertion: vacancy 0009 -> 0010, 0010 is the leaf
• runtime serializer/normalization/final-state validation assertions: passed
• python -m compileall: passed
• git diff --check: passed

Local PostgreSQL was unavailable, so targeted DB tests could not run separately on Windows; every targeted vacancy/workspace test is included in and passed by the full PostgreSQL CI suite. The ordinary local manage.py check is blocked by the missing Windows WeasyPrint gobject-2.0-0 system library; Django system checks ran successfully as part of the Linux CI test command.

Scope confirmation

Changed files are limited to vacancy model/constants/serializers/migration/tests and one project workspace vacancy contract test. React, Angular/frontend, workflows, Docker/deploy, infrastructure, and unrelated backend modules are unchanged.