Grid/contacts unification step 1: real contact_id link + grid as front door (v0.1.0:52)

Structural fix for the duplicate-people class of bug: instead of matching a grid
contact "pill" to a contacts row heuristically by name/email (which drifted and
caused the 1406 double-count), link them by id.

Backend:
- Migration 0004: fundraising_contacts.contact_id (additive, nullable, logical FK
  to contacts(id)) + index. Paired down migration.
- sync_fundraising_relational now stores the id that _upsert_contact_from_fundraising
  already returns, so every grid contact carries its contacts-table id.
- _backfill_grid_contact_ids: one-time, idempotent backfill on startup (re-runs the
  grid sync once if any row lacks contact_id), so existing data links immediately.
- entity_resolution: grid pass prefers the explicit contact_id link (match_kind
  'grid_link') over heuristic email / name+investor, guarded by a PRAGMA check so
  older DBs without the column still work.

Frontend:
- Fundraising grid "+ Row" -> "+ Investor" (clear, single investor entry point).
- Contacts page: the "+ Add Contact" trigger is replaced by a pointer to the grid;
  the page is now a read/search/edit view (ContactDetailPanel still edits all
  fields). New people are added from the grid. No contact data is removed.

Tests: backend/ingest/test_entity_resolution.py extended (explicit-link case, 11/11)
and a new backend/test_grid_contact_link.py integration test (init_db applies 0004,
sync populates contact_id to the right contact, re-sync is idempotent). py_compile +
frontend html.parser clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/index.html

@@ -3534,7 +3534,7 @@
                                 value={search}
                                 onChange={(e) => setSearch(e.target.value)}
                             />
-                            <button onClick={() => setShowForm(true)}>+ Add Contact</button>
+                            <span style={{ fontSize: '12px', color: '#8a93a6', alignSelf: 'center' }}>New people are added from the Fundraising Grid; click anyone here to view or edit.</span>
                         </div>
 
                         {loading ? (
@@ -7126,7 +7126,7 @@
                                 <option value="">All Leads</option>
                                 {teamMembers.map((m) => <option key={m} value={m}>{m}</option>)}
                             </select>
-                            <button onClick={addRow}>+ Row</button>
+                            <button onClick={addRow}>+ Investor</button>
                             <button className="button-secondary" onClick={() => setShowViewModal(true)}>+ Save View</button>
                             <button className="button-secondary" onClick={toggleRowDensity}>{rowDensity === 'compact' ? 'Expand Rows' : 'Compact Rows'}</button>
                             <button className="button-secondary" onClick={() => setShowColumnVisibilityModal(true)}>Columns</button>