Add OpenAPI docs, frontend, migrations, and API updates

- OpenAPI: add missing endpoints (add-from-url, subscriptions, public availability)
- OpenAPI: CalendarSubscription schema, Subscriptions tag
- Frontend app
- Migrations: count_for_availability, subscriptions_sync, user_preferences, calendar_settings
- Config, rate limit, auth, calendar, booking, ICS, availability, user service updates

Made-with: Cursor

frontend/node_modules/.bin/baseline-browser-mapping

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+../baseline-browser-mapping/dist/cli.cjs

frontend/node_modules/.bin/browserslist

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+../browserslist/cli.js

frontend/node_modules/.bin/esbuild

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+../esbuild/bin/esbuild

frontend/node_modules/.bin/jiti

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+../jiti/lib/jiti-cli.mjs

frontend/node_modules/.bin/jsesc

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+../jsesc/bin/jsesc

frontend/node_modules/.bin/json5

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+../json5/lib/cli.js

frontend/node_modules/.bin/nanoid

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+../nanoid/bin/nanoid.cjs

frontend/node_modules/.bin/parser

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+../@babel/parser/bin/babel-parser.js

frontend/node_modules/.bin/rollup

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+../rollup/dist/bin/rollup

frontend/node_modules/.bin/semver

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+../semver/bin/semver.js

frontend/node_modules/.bin/tsc

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+../typescript/bin/tsc

frontend/node_modules/.bin/tsserver

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+../typescript/bin/tsserver

frontend/node_modules/.bin/update-browserslist-db

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+../update-browserslist-db/cli.js

frontend/node_modules/.bin/vite

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+../vite/bin/vite.js