feat(lawful-basis): emit lawful basis and external media on every platform

[howethomas]

Stacked on #11 (which is stacked on #10). Base is that branch, so this diff shows only this change. Merge in order.

The shared core already supported a lawful basis attachment and external media, but only Telnyx turned them on. Asterisk, Bandwidth, FreeSWITCH and Twilio emitted vCons with no lawful basis and always inlined or referenced media.

Changes

• All four remaining webhook factories now build the media publisher and lawful basis from config and pass them to their builder, the same block Telnyx already used. No signature-validation code touched.
• Lawful basis attachment shape, targeting draft-ietf-vcon-vcon-core-04. vcon-lib 0.9.6's add_lawful_basis_attachment emits body as an object and omits party/dialog/mediatype. core/lawful_basis.py sets party: 0, dialog: 0, and mediatype: "application/json", and leaves body as the object vcon-lib already produces: under -04 §2.3.2 (CDDL body: any), body for encoding: "json" is the raw JSON value, not a json.dumps string. (An earlier revision of this PR stringified it against a stale schema fork that typed body as a string; that was wrong for -04 and has been reverted.) Result: purpose: "lawful_basis", encoding: "json", object body, "lawful_basis" in extensions.
• Unset LAWFUL_BASIS behaves as before: a warning and no attachment. No default basis in code.
• Every attachment now carries start, party, dialog, and mediatype, which draft-ietf-vcon-vcon-core-04's Attachment Object requires (mediatype whenever body is present). Missing values are backfilled in core/base_builder.py with the vCon's created_at, index 0, and application/json respectively; existing values are never overwritten.
• README common-configuration table lists LAWFUL_BASIS* and MEDIA_* as applying to every platform, and documents that JSON-encoded attachment bodies are raw values under -04, not json.dumps strings.

Tests

15 new tests: per platform, lawful basis configured (exact attachment shape, including mediatype), lawful basis unset (no attachment, warning logged), and MEDIA_BACKEND=filesystem (dialog has url + content_hash, no inline body); plus required attachment fields (start/party/dialog/mediatype on every attachment). Full stack: 539 passed.

Schema validation

One sample vCon per platform, built with LAWFUL_BASIS=consent and MEDIA_BACKEND=filesystem from test fixtures, validated with jsonschema against the working group's current schema (vendored at vcon-adapter-template's tests/schema/vcon_json_schema.json, WG main @ fdcf2f5, matching the -04 appendix):

asterisk   VALID
bandwidth  VALID
freeswitch VALID
twilio     VALID
telnyx     VALID

All five pass. The start/party/dialog and mediatype gaps are closed, and retargeting to -04 (raw JSON body, not a string) also resolves what looked like a second defect in the tags attachment: its body is a JSON array from vcon-lib's add_tag, which is a valid body value under -04's body: any — it only failed against the stale schema fork's body: string constraint.

One remaining, separate defect: MEDIA_BACKEND=embed (the default) still fails schema validation — dialog[0].encoding is "base64", which is not in the schema's ["base64url", "json", "none"] enum. That's in the inline-audio path in core/base_builder.py, unrelated to lawful basis or attachments, tracked separately as CON-1100.

Refs CON-1083

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