Add a new report asset to this Bruin pipeline: assets/posthog_reports/account_plan_changes.sql.

Grain: one row per company.

Source it from posthog_stage.persons and the latest month in posthog_reports.account_engagement_monthly. For each account:

• initial_plan - the plan held by the account's earliest-signed-up person, using signup_date to pick them
• current_plan - carried from posthog_stage.accounts.plan
• movement - upgraded, downgraded, unchanged, or unknown when either plan is missing, comparing the two on the ordinal free < pro < enterprise
• latest_engagement_score - the account's engagement_score from the most recent activity_month in account_engagement_monthly, null if the account has no engagement history yet

Match the conventions already used in assets/posthog_reports/:

• type: bq.sql with materialization: type: table, the same as the other reports
• an @bruin header with a description, depends, tags, and full columns with types, descriptions, and checks
• not_null and unique on company, and a custom check that movement is never unknown for an account whose initial_plan and current_plan are both set
• follow the plan-ordinal pattern already used in assets/posthog_stage/accounts.sql rather than inventing a new one

Show me the SQL before writing the file. After I approve, write it, run bruin validate, then bruin run assets/posthog_reports/account_plan_changes.sql and show me the movement counts.