Port the explore-timeline server foundation to main

The live site's fast merged cross-source timeline feed and the Explore over-time chart run on deploy-branch-only server code that was never merged to main: explore-timeline-substrate.ts (dual-dialect merged-timeline queries), the rs.explore.timeline operation, the GET /ref/explore/records route, and the semantictime column plus its idxrecordssemantictime / idxpgrecordssemantictime expression index. On fresh origin/main none of these exist: the records table carries only emittedat (ingest time), and the only timeline reader is GET /_ref/records/timeline (the ref-records-timeline operation), which orders by ingest time rather than authored/semantic time.

main already has the dual-dialect storage plumbing this port needs — the storage-backend.js abstraction, isPostgresStorageBackend(), the records table, the emittedat column, and idxrecordslookup. What it lacks is the semantic-time layer: the semantictime column, its expression index, the explore-timeline-substrate.ts merged-timeline queries, the rs.explore.timeline operation, and the GET /_ref/explore/records route. Those live only on the deploy branch, so the live fast feed runs on un-merged server code. This change ports that layer onto main verbatim in contract.

The reference records storage SHALL carry a semantictime column on the records table on both the SQLite and Postgres backends, declared TEXT NOT NULL DEFAULT ''. The column SHALL be added on a pre-existing, possibly non-empty records table by an idempotent migration that performs no bulk UPDATE of existing rows: on SQLite via a guarded ALTER TABLE records ADD COLUMN semantictime TEXT NOT NULL DEFAULT '' (guarded by a hasTableColumn check), and on Postgres via ALTER TABLE records ADD COLUMN IF NOT EXISTS semantictime TEXT NOT NULL DEFAULT ''. Existing rows SHALL retain '' and SHALL be read through COALESCE(NULLIF(semantictime, ''), emittedat), so a row without a real semantic value falls back to its emittedat and is never mis-attributed. New and re-emitted rows SHALL receive a real semantictime at write time via the record upsert (ON CONFLICT ... DO UPDATE SET semantictime = excluded.semantic_time). Re-running the migration SHALL be a no-op on both backends.