feat(examples): MediaBuy.invoice_recipient first-class column (3.1 readiness)

[bokelley]

Closes #378

AdCP 3.1 surfaces media_buy.invoice_recipient as a per-buy billing entity override. Previously it was buried in request_snapshot only — not queryable, not enforceable, and not surfaced in responses. This PR promotes it to a first-class SQLAlchemy column and wires it through the full create → update → list lifecycle in the v3 reference seller.

Changes:

• models.py: adds nullable invoice_recipient JSON column to MediaBuy; updates module docstring to note 3.1-readiness alongside Account
• platform.py: extracts and persists invoice_recipient in create_media_buy; propagates it (including explicit-null clearing via model_fields_set) in update_media_buy; adds get_media_buys implementation that queries the column and strips write-only bank via _project_invoice_recipient(); uses dict-based GetMediaBuysResponse.model_validate() to avoid importing generated_poc directly
• tests/test_smoke.py: two new tests — ORM column presence check and BusinessEntityResponse bank-stripping round-trip
• README.md: updates the Spec versioning section to enumerate invoice_recipient alongside the other 3.1-ready columns

What was tested:

• pytest examples/v3_reference_seller/tests/test_smoke.py::test_invoice_recipient_round_trips_without_bank — passes
• pytest tests/ -q — 3097 passed, 1 pre-existing failure (TLS integration test unrelated to this change)
• ruff check examples/v3_reference_seller/src/ — all checks passed
• mypy src/adcp/ --ignore-missing-imports — no issues found in 744 source files

Nits (not fixed, surfaced for reviewer):

• status_filter duck-typing via hasattr(..., "root") is fragile if generated shape changes; prefer isinstance(status_filter, RootModel) with a comment
• test_smoke.py test replicates _project_invoice_recipient logic inline rather than importing the function directly

Pre-PR review:

• code-reviewer: approved — fixed str(status_filter) → .value blocker (silent zero-row bug), fixed generated_poc import violation via dict+model_validate, fixed currency/total_budget null fabrication, fixed model_fields_set for clearable update; 2 nits noted above
• dx-expert: approved — _project_invoice_recipient follows to_account_response pattern, migration comment in docstring addresses adopter gotcha, generated_poc import violation resolved via dict approach

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