feat(casts): add root_parent tracking for thread identification

[christopherwxyz]

Summary

• Add root_parent_hash, root_parent_fid, root_parent_url columns to casts table for O(1) thread identification
• Resolve root parent during cast insertion via DB lookup or Hub traversal
• Add backfill worker to populate existing casts
• Leave root_parent NULL when chain is broken (never incorrectly assume non-root is root)

Changes

Migration (006_add_root_parent_columns.sql)

• Adds 3 nullable columns: root_parent_fid, root_parent_hash, root_parent_url
• Creates partial indexes for efficient lookups

Cast Processing (src/processor/database.rs)

• resolve_root_parent(): Checks DB for parent's root, falls back to Hub traversal
• resolve_root_from_hub(): Traverses parent chain via gRPC to find true root
• Updated add_cast() to populate root_parent columns

Batch Operations (src/database/batch.rs)

• Added parent_fid, root_parent_* fields to CastInsert
• Updated bulk_insert_casts() for new columns

Backfill (src/backfill/root_parent.rs)

• RootParentBackfill worker for existing casts
• backfill_url_parents() for simple URL parent casts
• Configurable batch size, max depth, delay

Semantics

Scenario	root_parent_* value
Root cast (no parent)	NULL
Reply with known root	Points to root cast
Reply with URL parent	root_parent_url set
Broken chain (parent missing)	NULL

Test plan

• Unit tests for extract_parent_from_proto()
• Clippy passes with no warnings
• All 199 tests pass
• Manual test with real Hub connection
• Run backfill on staging data