Communications tab: show matched investors only (v0.1.0:81)

The email-activity panel surfaced every captured message, including cold/
unknown-sender email with no investor association. Gate query_email_activity
on EXISTS(email_investor_links) so the panel shows only email tied to a known
investor/contact. Capture is unchanged — unmatched email is still stored
(metadata-only) and will appear automatically if its sender is later added as
an investor; this is a read-side filter only.

Graveyard investors are unaffected (their email has a link), so they remain
visible/searchable as an audit surface, hidden only from the filter picker.

backend/email_integration/db.py

@@ -405,6 +405,10 @@ def query_email_activity(conn: sqlite3.Connection, *, investor_id: Optional[str]
     investor (matched fundraising investor) and/or mailbox, with free-text search
     over subject/snippet/sender. Returns the email rows plus the filter facets.
 
+    Matched-only: the panel shows ONLY email that links to a known investor/contact
+    (an `email_investor_links` row exists). Unmatched cold/unknown-sender email is
+    still captured for completeness but is never surfaced here.
+
     Soft-delete: an email is live only if it still has a non-tombstoned per-mailbox
     sighting (`email_account_messages.deleted_at IS NULL`) — the `emails` row itself
     carries no deleted_at, so deletion lives on the sighting. Direction is decided at
@@ -418,7 +422,10 @@ def query_email_activity(conn: sqlite3.Connection, *, investor_id: Optional[str]
     own.discard("")
 
     where = ["EXISTS (SELECT 1 FROM email_account_messages eam "
-             "WHERE eam.email_id = e.id AND eam.deleted_at IS NULL)"]
+             "WHERE eam.email_id = e.id AND eam.deleted_at IS NULL)",
+             # Matched-only: surface email that links to a known investor/contact.
+             # Unmatched (unknown-sender) email is captured but never shown here.
+             "EXISTS (SELECT 1 FROM email_investor_links l WHERE l.email_id = e.id)"]
     params: list = []
     if account_id:
         where.append("EXISTS (SELECT 1 FROM email_account_messages eam "