[security] Validate tycoon.yml identifiers/paths (charset + project-root containment) (Medium)

[db-tycoon-stephen]

Summary

Add a Pydantic validation layer to tycoon.yml so that identifiers and paths from a shared/untrusted config cannot reach SQL strings or the filesystem unchecked. This is the root-cause fix backing the SQL-injection and path-traversal findings.

Evidence

• src/tycoon/project.py:113-130, 251-292 — SourceConfig.schema_name, source map keys (source_name), dbt_project_dir, rill_dir are all plain str, no constraints.
• source_name flows into file paths: output_dir / f"{model_name}.sql", dbt_project_dir / "models" / "staging" / source_name (commands/explore.py:142, dbt_generator.py:267) → path traversal (../../...) on tycoon data analyze.
• source_name/table_name flow into the dbt template {{ source('{source_name}', '{table_name}') }} (dbt_generator.py:113) → break-out into generated SQL.
• dbt_project_dir/rill_dir accept absolute / .. paths, .resolve()d with no containment check (config.py:78-87).

Fix

• Constrain source keys and schema_name to ^[A-Za-z_][A-Za-z0-9_]*$.
• After resolving dbt_project_dir/rill_dir, assert resolved.is_relative_to(self.root) unless the user interactively opted into an external path.
• schedule.py's _NAME_RE is the model to mirror.

Severity: Medium (enables the High SQL-injection / traversal findings)

[db-tycoon-stephen]

Mirrored in Jira: PTC-37 (Epic: PTC-31 — Security hardening). Dev work continues here.