feat: change tracker email template

[Alessandro100]

Summary:

This email is still going to junk in Outlook which is relevant to: MobilityData/mobilitydatabase-web#121

closes: https://github.com/MobilityData/product-tasks/issues/198

Introduces the email template to use for notification changes based on the gtfs diff schema. Also includes features not included in the schema like: validation diff, feature diff, breaking changes, and suspicious changes

Expected behavior:

When we are ready to send emails regarding the gtfs change notifications, this email template will format it nicely

Testing tips:

At the root of the mobility-feeds-api run

cd api/src

python - <<'EOF'
from pathlib import Path
from jinja2 import Environment, FileSystemLoader, select_autoescape

def thousands_sep(n):
    if n is None or n == "": return "—"
    try: return "{:,}".format(int(n))
    except: return "—"

env = Environment(
    loader=FileSystemLoader("email_templates"),
    autoescape=select_autoescape(["html", "jinja2"]),
)
env.filters["thousands_sep"] = thousands_sep

context = {
    "notification_date": "June 11, 2026",
    "subscriptions_url": "https://mobilitydatabase.org/subscriptions",
    "feeds": [{
        "feed_id": "mdb-2126",
        "feed_name": "Sample Transit Agency",
        "feed_url": "https://mobilitydatabase.org/feeds/mdb-2126/diff",
        "unsubscribe_url": "https://mobilitydatabase.org/feeds/mdb-2126/unsubscribe",
        "notification_date": "June 11, 2026",
        "base_feed_version": "2026-05-01", "new_feed_version": "2026-06-11",
        "total_changes": 42, "files_modified_count": 3,
        "breaking_changes": [{"type": "REMOVED ENTITY", "file": "trips.txt", "description": "12 trips removed."}],
        "suspicious_changes": [{"type": "LARGE ENTITY COUNT DELTA", "file": "stop_times.txt", "description": "Count dropped >20%."}],
        "diff_files": [
            {"file_name": "trips.txt", "rows_added_count": 5, "rows_deleted_count": 12, "rows_modified_count": 3},
            {"file_name": "calendar.txt", "rows_added_count": 2, "rows_deleted_count": 0, "rows_modified_count": None},
        ],
        "features_changed": [{"name": "Stops Wheelchair Accessibility", "change": "Added"}],
        "validation_changes": [
            {"change_type": "New", "notice_code": "missing_required_field", "severity": "ERROR", "older_count": None, "latest_count": 3},
            {"change_type": "Fixed", "notice_code": "invalid_url", "severity": "WARNING", "older_count": 2, "latest_count": None},
        ],
    }],
}

Path("/tmp/preview.html").write_text(
    env.get_template("change_tracker_email.html.jinja2").render(**context)
)
print("Saved to /tmp/preview.html")
EOF

open /tmp/preview.html

which will open up a preview. You can modify the values accordingly

Follow up tickets

• Link the "View Full Report" buttons to actual links
• Link the "Manage Notifications" to an actual page

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[Copilot]

Pull request overview

Adds a new Jinja2 HTML email template intended to format GTFS change-tracker notifications (diff summary + breaking/suspicious changes + feature and validation diffs) for eventual outbound email delivery.

Changes:

• Introduces change_tracker_email.html.jinja2 with a multi-feed table-of-contents and per-feed sections.
• Adds rendering for diff summary stats, breaking/suspicious change callouts, feature changes, and validation report diffs.
• Includes email-client specific markup (e.g., Outlook/VML button) and responsive styles.