feat(schema): add DOOH structured selling-unit fields to placements

[garvitkaushik-123]

Summary

Implements #5537 — adds structured DOOH selling-unit metadata to placements so buyers can evaluate screen characteristics during product discovery.

• dooh_placement_attributes object on both placement.json and placement-definition.json with four optional fields:
  • slot_duration_seconds — duration of one ad slot
  • loop_duration_seconds — full loop rotation (canonical source, supersedes pricing-layer field)
  • screen_resolution — physical { width, height } in pixels
  • motion — full_motion, partial_motion, or static (new dooh-motion-type enum)
• identifiers[] array on both placement schemas — reuses the { type, value } pattern from property identifiers for DOOH venue_id, screen_id, openooh_venue_type, etc.
• dooh-motion-type.json enum — full_motion, partial_motion, static
• identifier-types.json title updated from "Property Identifier Types" to channel-neutral "Identifier Types"
• flat-rate-option.json loop_duration_seconds description updated with superseded note
• Docs — DOOH placement attributes section added to media-products.mdx

Design decisions

• Follows the existing channel-overlay pattern (video_placement_types, audio_distribution_types, etc.) — new fields sit alongside those in the same position
• additionalProperties: true on dooh_placement_attributes to allow future field additions without breaking changes
• identifiers[] is channel-neutral by design — reuses the same enum already used for property identifiers
• motion enum values align with industry conventions (OpenOOH/OAAA terminology)

Test plan

• All 4232 unit tests pass
• TypeScript typecheck passes
• Schema build (build:schemas) succeeds
• Verify DOOH placement example validates against updated schema

Closes #5537

🤖 Generated with Claude Code

[aao-release-bot]

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