Add a PipesHub connector

Follow CONNECTOR.md and the documents it links in docs/connectors/. This skill is the order of work and the points where you must stop; the documents hold the detail. Where they disagree with the code, the code is right: say so, and fix the document in the same PR.

1. Understand before you design

1. Read CONNECTOR.md, then docs/connectors/01-high-level-design.md and 02-low-level-design.md.
2. Pick the reference connector of the same shape from the table in docs/connectors/05-step-by-step.md and read its connector class, client and behaviour tests.
3. Read the vendor's API documentation for everything listed under "Find out yourself" in docs/connectors/04-intake-questions.md. Note the documentation page for each fact.

2. Ask, once, with recommendations

Send the human one message with the questions in 04-intake-questions.md that the documentation did not answer, and the decisions in docs/connectors/03-decisions.md that need them. For each, give your recommendation and what you found, so they can confirm. Always ask for a test account and credentials, and say what cannot be verified without one.

Do not write code until the human has answered. If they cannot provide a test account, carry on with unit tests against fakes, and say in the PR that the connector was not run against the real source.

3. Write the design into the PR description

Before code: the "Connector intake" section, each decision from 03-decisions.md with its reason, and the permission mapping per object type from docs/connectors/06-permissions.md. Show it to the human if any part of the permission mapping errs towards over-sharing or hides content the source shows.

4. Build in phases

Work through docs/connectors/05-step-by-step.md phase by phase. Run each phase's check and do not start the next phase until it passes. While writing any API call, apply docs/connectors/07-errors-and-logging.md. While writing any record or group, apply 06-permissions.md.

Stop and ask the human when:

• the source's sharing cannot be expressed without over-sharing;
• a needed API scope may not be grantable, or needs a paid tier;
• the live API contradicts its documentation;
• you would need to change shared code under backend/python/app/connectors/core/, backend/python/app/models/ or the graph providers.

5. Finish

1. Run the greps in 07-errors-and-logging.md on the new connector and its client, and read every hit.
2. Run the checks in docs/connectors/08-testing.md: the contract tests, the connector's tests, the whole unit suite, ruff on the changed files.
3. Go through CONNECTOR_CHECKLIST.md item by item, and copy it, ticked, into the PR description. For each unticked item, say why.
4. Report to the human what was verified against the real source, what only against fakes, and what is known not to work.