Add birthday email reminder functionality

Implements birthday reminder emails that are sent daily at 9 AM for birthdays
occurring within the next 7 days. Integrates with existing birthday.py module
and mail system to send notifications with appropriate timing-based subjects.

Fixes #171

🤖 Generated with [Claude Code](https://claude.ai/code)

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

update.py

@@ -12,6 +12,7 @@ import deepdiff
 import flask
 import requests
 
+import agenda.birthday
 import agenda.bristol_waste
 import agenda.fx
 import agenda.geomob
@@ -358,6 +359,66 @@ def update_gandi(config: flask.config.Config) -> None:
         out.write(r.text)
 
 
+def check_birthday_reminders(config: flask.config.Config) -> None:
+    """Check for upcoming birthdays and send email reminders."""
+    today = date.today()
+    data_dir = config["PERSONAL_DATA"]
+    entities_file = os.path.join(data_dir, "entities.yaml")
+
+    if not os.path.exists(entities_file):
+        return
+
+    # Get upcoming birthdays (next 7 days)
+    birthdays = agenda.birthday.get_birthdays(today, entities_file)
+    upcoming_birthdays = []
+
+    for birthday in birthdays:
+        days_until = (birthday.date - today).days
+        if 0 <= days_until <= 7:
+            upcoming_birthdays.append((birthday, days_until))
+
+    if not upcoming_birthdays:
+        return
+
+    # Group birthdays by days until
+    birthday_groups = {}
+    for birthday, days_until in upcoming_birthdays:
+        if days_until not in birthday_groups:
+            birthday_groups[days_until] = []
+        birthday_groups[days_until].append(birthday)
+
+    # Send reminder emails
+    for days_until, birthdays in birthday_groups.items():
+        if days_until == 0:
+            subject = "🎂 Birthday Today!"
+        elif days_until == 1:
+            subject = "🎂 Birthday Tomorrow!"
+        else:
+            subject = f"🎂 Birthday in {days_until} days"
+
+        body_lines = ["Birthday Reminder:\n"]
+        for birthday in birthdays:
+            date_str = birthday.date.strftime("%A, %B %d, %Y")
+            if days_until == 0:
+                body_lines.append(f"Today: {birthday.title} - {date_str}")
+            elif days_until == 1:
+                body_lines.append(f"Tomorrow: {birthday.title} - {date_str}")
+            else:
+                body_lines.append(f"{date_str}: {birthday.title}")
+
+        body_lines.append(
+            f"\nView all birthdays: https://edwardbetts.com/agenda/birthdays"
+        )
+
+        body = "\n".join(body_lines)
+
+        if sys.stdin.isatty():
+            print(f"Birthday reminder: {subject}")
+            print(body)
+
+        agenda.mail.send_mail(config, subject, body)
+
+
 def main() -> None:
     """Update caches."""
     now = datetime.now()
@@ -376,6 +437,10 @@ def main() -> None:
 
         update_thespacedevs(app.config)
 
+        # Check for birthday reminders daily at 9 AM
+        if hour == 9:
+            check_birthday_reminders(app.config)
+
 
 if __name__ == "__main__":
     main()