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/app/schemas/user.py

@@ -129,6 +129,9 @@ class UserPreferencesUpdate(BaseModel):
     # Personal Jira API token (Atlassian token) — write-only, stored encrypted at rest.
     # Set to empty string "" to clear the token.
     jira_api_token: str | None = None
+    # Atlassian email for Jira auth — overrides account email.
+    # Set to empty string "" to clear (falls back to account email).
+    jira_email: str | None = None
 
 
 class UserOut(BaseModel):
@@ -144,6 +147,7 @@ class UserOut(BaseModel):
     last_login: datetime | None = None
     notification_preferences: dict | None = None
     jira_account_id: str | None = None
+    jira_email: str | None = None
     # Never return the raw token — just indicate whether it is configured.
     jira_token_set: bool = False