[phornstein]

Phase 2 of #104: the backend for drafts and structured errors.

Behaviour (as decided in #104):

• Drafts. A source that isn't complete yet saves as a draft instead of being refused. Only a spec that doesn't parse, an invalid id or name, or a disallowed file path stops a save.
• Drafts can't be enabled. This is enforced in the API and in the store. Enable checks the source again and stores what it finds, so a draft whose problem was resolved elsewhere (for example a schema published since) can be enabled without being saved again.
• An enabled source is saved only when complete. Otherwise the save is refused and the running source is kept. This is enforced inside the store's write, so a concurrent enable can't get around it.
• Every problem at once. A refusal is a 422 with {error, problems: [{kind: invalid|incomplete, stage, field?, message}]}. stage is source, decode, map, static_join, registry, affiliation, filter, tracker, throttle or output, and null for the source as a whole. error is still the first problem's message, so existing callers keep working.
• Rows carry their status. Each row has draft and problems, kept in a new column (migration 0020, SQLite schema 20). Sources saved before this migrate as complete.
• Empty transport settings are named: host, port, URL, bind address, topic, method or path. Every mapping rule's problems are reported, not just the first.
• Config backups carry drafts. An enabled source in an imported file must be complete, and every problem it has is listed.

Code

• SourceSpec::problems() (ot-source) does all the checking. validate() returns its first result, as before; validate_against is removed.
• TransportConfig::missing() and MappingSpec::problems() are new.

UI. Nothing visible changes. The client types gain draft, problems and SourceProblem, and ApiError.problems is kept for the editor in phase 4. The current Add Source wizard now updates a source it has already saved as a draft, instead of trying to create it again (which returned 409).

Review. /code-review high found 10 issues. All 10 are fixed in the second commit.

Gate (dev server): cargo fmt --check and cargo clippy --workspace --all-targets -D warnings pass. cargo test --workspace passes except nodes_share_one_picture_under_one_number, the date bug fixed by #106. UI: tsc -b passes, oxlint has 0 errors, vitest 58/58.

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