test: add unit tests, API schema snapshot and CI test pipeline

[TwoDCube]

Step 1 of 3 in adding a test suite (1: unit tests + schema snapshot + CI, 2: MySQL/Redis integration and response snapshot tests, 3: one PR per bug found).

What changes

Library/binary split. src/lib.rs now holds Config, AppState, app() and run(); src/main.rs only reads the environment, connects and serves. Nothing in the library reads env vars or opens connections itself: graphql::build_schema takes the persisted-query cache as an argument instead of connecting to Redis, and AppState::new takes the schema, database and config.

Logic moved out of resolvers, extraction only, no behaviour change:

• utils/date_range.rs: month_range (archive), calendar_grid_range (events), iso_week_range (canteen)
• colleagues::compare_names: Hungarian collation, ignoring a Dr. prefix
• posts::image_urls: turns the images JSON (array or object) into URLs

Tests: 53 unit tests, some with proptest invariants:

• date ranges: month ends, December → January, ISO week 1 and week 53
• Date/DateTime scalars and PostCursor encode/decode round-trips
• connection page info: pins the current hasPreviousPage = "older posts exist" meaning, since website_frontend2 requests these fields
• Hungarian digraph and long-vowel ordering; image URLs; config defaults and errors

API contract snapshot. insta snapshots schema.sdl() (src/graphql/snapshots/). Adding, renaming or removing a field, or changing its type or nullability, now appears as a diff in the PR.

CI (.github/workflows/ci.yml):

• runs on pull_request and pushes to master; branch pushes without a PR no longer trigger it
• replaces the deprecated actions-rs/* actions with dtolnay/rust-toolchain, Swatinem/rust-cache@v2 and actions/checkout@v7
• tests run with cargo-nextest, plus doctests
• clippy uses --all-targets, so test code is linted too
• job names Build/Format/Clippy/Test are unchanged, because branch protection requires them

Docs: docs/testing.md covers how to run the tests, update snapshots and handle known bugs.

Bugs found (ignored tests, fixed in follow-up PRs)

Test	Bug
calendar_grid_ends_after_the_sunday_of_the_last_week (+ proptest)	events(year, month) stops one day before the end of the calendar grid, so events on the grid's last Sunday are missing. When the next month starts on a Monday it adds a spare 6 days instead.
calendar_grid_does_not_panic_at_the_earliest_representable_month	events(year: -262143, month: 1) overflows chrono and panics, returning HTTP 500.
parses_the_format_it_emits	The DateTime scalar documents and emits YYYY-MM-DD HH:MM:SS, but its parser only accepts YYYY-MM-DDTHH:MM:SS.

cargo nextest list --run-ignored only lists them.

Verification

• cargo fmt --check passes; cargo clippy --locked --all-targets -- -D warnings is clean.
• cargo nextest run --all-targets: 53 passed, 4 skipped (the known bugs). A second run without INSTA_UPDATE passes, so the snapshot is stable.
• cargo deny check bans licenses sources passes with the new dev-dependencies (insta, proptest). The advisories findings (h2, rustls, proc-macro-error2) already exist on master.

🤖 Generated with Claude Code

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