Adopt the Pipeline: grid-driven opportunities link (v0.1.0:87)

The fundraising grid (canonical) now drives the classic opportunities
Pipeline board, instead of the board being a disconnected second data-entry
surface. An "Add to Pipeline" row action creates a durably-linked opportunity
via the new opportunities.fundraising_investor_id (migration 0005, additive +
reversible), reusing the grid's already-synced contact — retiring the
POST /api/contacts side-door — and mapping the grid lead to the opp owner.

Ownership is split so the two stay reconciled: the grid owns whether the link
exists and the seed; the board owns stage/probability/owner. The link endpoint
is idempotent (one live opp per investor; a re-link never reseeds funnel
fields). "Is in pipeline?"/"what stage?" are derived from a live opp join and
injected as read-only grid columns on read, stripped on write, so they never
persist or dirty the autosave. Remove-from-pipeline soft-deletes the opp and
leaves the grid row fully intact; deleting an investor from the grid archives
its orphaned opp.

Also fixes the standing soft-delete leak in handle_pipeline_report and the
dashboard pipeline aggregates, which counted tombstoned opportunities.

Tests: backend/test_grid_pipeline_link.py (link/idempotent/round-trip/guards/
unlink-intact/re-link/orphan/aggregates); 28/28 suite green, render-smoke green.

backend/migrations/0005_grid_pipeline_link.down.sql

@@ -0,0 +1,7 @@
+-- Reversal of 0005_grid_pipeline_link.sql (manual; .down files are never auto-applied).
+--
+-- SQLite < 3.35 cannot DROP COLUMN. The added column is nullable and ignored by any code
+-- path predating it, so leaving it in place is harmless. The index drops freely. On
+-- SQLite >= 3.35 the column itself may also be dropped.
+DROP INDEX IF EXISTS idx_opportunities_fr_investor;
+-- ALTER TABLE opportunities DROP COLUMN fundraising_investor_id;  -- SQLite >= 3.35 only