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/migrations/0007_pipeline_stages_v2.down.sql

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+-- Reversal of 0007_pipeline_stages_v2.sql (manual; .down files are never auto-applied).
+--
+-- BEST-EFFORT: the 6->4 stage collapse is lossy and cannot be perfectly inverted (the
+-- pattern other .down files here share -- e.g. 0005 cannot DROP COLUMN on old SQLite). It
+-- restores VALID legacy 6-stage values, choosing a representative for each collapsed pair:
+--   engaged    was outreach OR meeting  -> 'meeting'        (representative)
+--   diligence  -> 'due_diligence'                            (exact)
+--   commitment was committed OR funded  -> 'committed'      (representative)
+-- Opportunities archived from the stray 'lost' value still carry stage = 'lost' but cannot be
+-- re-identified as "archived by this migration" vs archived for other reasons, so they are
+-- left archived; un-archive (clear deleted_at) manually if a rollback truly needs them back.
+UPDATE opportunities SET stage = 'meeting'        WHERE stage = 'engaged';
+UPDATE opportunities SET stage = 'due_diligence'  WHERE stage = 'diligence';
+UPDATE opportunities SET stage = 'committed'      WHERE stage = 'commitment';

backend/migrations/0007_pipeline_stages_v2.sql

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+-- Pipeline funnel v2 — collapse the inherited 6-stage opportunity funnel into the locked
+-- 4-stage per-investor funnel: lead -> engaged -> diligence -> commitment, terminal at
+-- commitment. See ROADMAP "Pipeline stages + investor flags/labels -- LOCKED SPEC" (2026-06-19)
+-- and server.PIPELINE_STAGES.
+--
+-- DATA-ONLY + DEPLOYMENT-STATE-INVARIANT (migrations guide): targets stage values
+-- structurally, so it is a no-op on a fresh DB (no opportunities) and remaps deterministically
+-- on a populated one.
+--   outreach, meeting   -> engaged      (a two-way conversation has begun; "meeting" was an
+--                                         activity, not a position, so it folds in here)
+--   due_diligence       -> diligence
+--   committed, funded   -> commitment   (terminal; post-commit $ lives in the grid fund cell,
+--                                         and fund admin owns post-commitment -- no "funded" stage)
+UPDATE opportunities SET stage = 'engaged'    WHERE stage IN ('outreach', 'meeting');
+UPDATE opportunities SET stage = 'diligence'  WHERE stage = 'due_diligence';
+UPDATE opportunities SET stage = 'commitment' WHERE stage IN ('committed', 'funded');
+
+-- The stray legacy 'lost' value is not in the new settable enum, and a lost deal is a dead
+-- deal: ARCHIVE (soft-delete) the opportunity rather than leave an un-settable stage on a live
+-- row. The grid investor row is left fully intact (the grid is canonical); graveyarding the
+-- investor stays a human action, never an auto-mutation (human-in-the-loop guardrail). The
+-- stage text is left as 'lost' on the archived row for provenance -- it is filtered out
+-- everywhere by deleted_at IS NULL.
+UPDATE opportunities SET deleted_at = datetime('now'), updated_at = datetime('now')
+    WHERE stage = 'lost' AND deleted_at IS NULL;