connector: add Tableau ingestor (BI / analytics) #1571

Description

Add a Tableau connector (BI / analytics) to semantica/ingest/.

Why: Lets Semantica see which datasets and fields the business actually consumes downstream, not just what exists upstream — closes the loop from source data to reporting semantics.

Scope

Pull workbook/datasource/field metadata via the Tableau REST API (tableauserverclient). Metadata only for phase 1, not full extract data.

Follow the existing connector pattern (see semantica/ingest/sap_ingestor.py, snowflake_ingestor.py, or salesforce_ingestor.py for reference):

• semantica/ingest/tableau_ingestor.py — a *Data result dataclass, a *Connector (auth + client), and a *Ingestor exposing export_as_documents so output plugs directly into GraphBuilder.
• Register the three classes as lazy imports in semantica/ingest/__init__.py (see the existing _LAZY_IMPORTS-style entries for SAP/Snowflake/Salesforce) — importing semantica.ingest must not import this connector's SDK eagerly.
• docs/integrations/tableau.md — usage doc, following docs/integrations/salesforce.md / docs/integrations/snowflake.md as a template. Add it to docs/docs.json navigation.
• tests/test_tableau_ingestor.py — unit tests with the client/SDK mocked, matching tests/test_sap_ingestor.py / tests/ingest/test_sap_ingestor.py.
• Any outbound HTTP call must go through request_with_ssrf_guard (see semantica/ingest/sap_ingestor.py) if this connector talks to a user-supplied endpoint.

Keep core lightweight — optional dependency required

This connector's SDK must not be a core dependency. Add it to pyproject.toml under [project.optional-dependencies]:

ingest-tableau = ["tableauserverclient>=0.25"]

and raise a clear ImportError (with the pip install "semantica[ingest-tableau]" hint) if the connector is used without the extra installed — see how db-salesforce / ingest-sap do this today in semantica/ingest/__init__.py.

Acceptance Criteria

• pip install semantica (core, no extras) still works and does not pull in tableauserverclient
• pip install "semantica[ingest-tableau]" installs the connector's dependency
• Connector follows the *Data / *Connector / *Ingestor shape and export_as_documents output contract used by existing connectors
• Docs page added and linked in docs/docs.json
• Unit tests with the external client mocked (no live-service calls in CI)

help wanted — open to any contributor. Comment here if you'd like to pick it up and a maintainer will assign it.