BigQuery schema gaps and orphaned social_post handles

[yav02]

An audit of db/schema.bq.sql, the staging schemas in db/staging/, and docs/DB_SCHEMA.md against the live dataset found several discrepancies.

1. Missing staging columns

db/schema.bq.sql defines extended_summary_he and generation_run_id on mk_issue_summary, but db/staging/stg_summaries.schema.json omits both. This causes bq load schema mismatches during summary ingestion.

Fix: add both columns to stg_summaries.schema.json and document them in docs/DB_SCHEMA.md.

2. Undocumented table

summary_generation_run exists in db/schema.bq.sql but is entirely absent from docs/DB_SCHEMA.md.

Fix: document the table and its clustering keys.

3. Stale issue counts in comments

db/schema.bq.sql and docs/DB_SCHEMA.md both state 7 issues in comments, while db/seed.bq.sql seeds 9. This is the same underlying drift tracked in the pipelines issue covering EXPECTED_ISSUE_COUNT; fix the comments as part of reconciling the taxonomy, not separately.

4. Orphaned handles in the live dataset

social_post contains roughly 36 rows where mk_id holds a raw account handle rather than a UUID, violating the foreign-key relationship the schema and docs/INGESTION_CONTRACT.md both assume.

Fix: a one-off statement mapping the affected handles to their mk.id, plus a change to the pipeline's handle resolver so unresolvable handles are dropped rather than written through as text.