feat(jira): add editable jira_email field per user

Users can now set a separate Atlassian email for Jira authentication
in Settings → Profile → Jira Integration. Falls back to the Aegis
account email when not set, so existing setups are unaffected.

- Migration b043: adds jira_email column to users table
- User model/schema: expose jira_email read/write
- jira_service: _effective_jira_email() uses jira_email ?? email
- Frontend: replaces read-only email display with editable input

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/b043_jira_email.py

@@ -0,0 +1,28 @@
+"""Add jira_email to users table.
+
+Allows each user to specify a separate email for Jira authentication,
+independent of their Aegis account email.
+
+Revision ID: b043
+Revises: b042
+Create Date: 2026-05-26
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "b043"
+down_revision = "b042"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "users",
+        sa.Column("jira_email", sa.String(255), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("users", "jira_email")