Pipeline funnel v2: 4-stage enum + migration 0007 + derived grid signals

Collapse the inherited 6-stage opportunity funnel to the locked 4-stage
per-investor funnel (lead -> engaged -> diligence -> commitment), terminal at
commitment. Migration 0007 remaps existing stage values (outreach/meeting ->
engaged, due_diligence -> diligence, committed/funded -> commitment) and
archives the stray 'lost' value (the grid row is left intact). Inject read-only
existing_investor (total_invested>0), last_activity_at, and staleness
(''/'aging'>=30d/'stale'>=60d) into the grid GET, stripped on write. Frontend:
4-stage chip tints + Pipeline board / opp-form / mock on the new enum.

The visible desktop existing-investor star + staleness recency column + the
Stale saved view are deferred to mobile Phase 3 (data is injected + test-locked
now, so that phase stays pure-frontend). Longshot was already retired by prior
cleanup -- no-op.

Tests: test_pipeline_stages_v2.py (migration remap + derivation boundaries) +
updated grid-pipeline-link / soft-delete / nl_query; 36/36 green, render-smoke
green, fresh-DB migrate clean.

backend/scripts/seed_synthetic.py

@@ -193,8 +193,8 @@ def main():
                 "expected_amount, probability, fund_name, description, next_step, owner_id, priority, updated_at) "
                 "VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?)",
                 (gen(), f"{org_name or person} — {fund_label}", cid, org_ids.get(org_name) if org_name else None,
-                 stage, amt if stage in ("committed", "funded") else 0, amt,
-                 {"lead": 10, "outreach": 25, "meeting": 40, "due_diligence": 60, "committed": 90, "funded": 100}[stage],
+                 stage, amt if stage == "commitment" else 0, amt,
+                 {"lead": 10, "engaged": 35, "diligence": 60, "commitment": 90}[stage],
                  fund_label, f"Potential {fund_label} allocation for {person}.",
                  random.choice(["Send deck", "Schedule call", "Await IC", "Send subdocs"]),
                  uid, random.choice(["low", "medium", "high"]), now()))