feat(events): recurring event series with occurrence sync

Add EventSeries (club-scoped): an RFC-5545 rrule + dtstart + duration and a
template (kind/title/location/opponent + audience M2M). Concrete Event rows
are materialised occurrences carrying a series FK plus detached/cancelled
flags; the series tracks excluded_dates (EXDATEs) and a generated_until
horizon watermark.

Recurrence service:
- occurrence_datetimes expands the rrule (via python-dateutil) up to a
  horizon, minus EXDATEs.
- generate_occurrences materialises missing rows, copies the template +
  audience (so attendance syncs through the existing signals), and is
  idempotent.
- cancel_occurrence adds an EXDATE and deletes (or soft-cancels) one
  occurrence so it isn't regenerated; detach_occurrence marks an occurrence
  as independently edited; propagate_series re-applies the template to
  non-detached future occurrences.
- extend_event_series management command rolls the horizon forward.

Register EventSeries in the admin and surface series/detached/cancelled on
the Event admin. Add python-dateutil. Full suite at 100% coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

uv.lock

@@ -20,6 +20,7 @@ dependencies = [
     { name = "django" },
     { name = "django-phonenumber-field", extra = ["phonenumbers"] },
     { name = "pillow" },
+    { name = "python-dateutil" },
     { name = "python-decouple" },
 ]
 
@@ -35,6 +36,7 @@ requires-dist = [
     { name = "django", specifier = ">=6.0.6" },
     { name = "django-phonenumber-field", extras = ["phonenumbers"], specifier = ">=8.4.0" },
     { name = "pillow", specifier = ">=12.3.0" },
+    { name = "python-dateutil", specifier = ">=2.9.0.post0" },
     { name = "python-decouple", specifier = ">=3.8" },
 ]
 
@@ -185,6 +187,18 @@ wheels = [
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 ]
 
+[[package]]
+name = "python-dateutil"
+version = "2.9.0.post0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "six" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/66/c0/0c8b6ad9f17a802ee498c46e004a0eb49bc148f2fd230864601a86dcf6db/python-dateutil-2.9.0.post0.tar.gz", hash = "sha256:37dd54208da7e1cd875388217d5e00ebd4179249f90fb72437e91a35459a0ad3", size = 342432, upload-time = "2024-03-01T18:36:20.211Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/ec/57/56b9bcc3c9c6a792fcbaf139543cee77261f3651ca9da0c93f5c1221264b/python_dateutil-2.9.0.post0-py2.py3-none-any.whl", hash = "sha256:a8b2bc7bffae282281c8140a97d3aa9c14da0b136dfe83f850eea9a5f7470427", size = 229892, upload-time = "2024-03-01T18:36:18.57Z" },
+]
+
 [[package]]
 name = "python-decouple"
 version = "3.8"
@@ -219,6 +233,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/2d/c7/c53e8dbff9c9dc4b7928773421ae294a5d28fcb8dcda1a089579d3a7e510/ruff-0.15.17-py3-none-win_arm64.whl", hash = "sha256:f3be1fbb34bcdfd146240d8fb92a709d4c2c8191348580a3c044ec60fa0b4456", size = 11355275, upload-time = "2026-06-11T17:54:43.635Z" },
 ]
 
+[[package]]
+name = "six"
+version = "1.17.0"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/94/e7/b2c673351809dca68a0e064b6af791aa332cf192da575fd474ed7d6f16a2/six-1.17.0.tar.gz", hash = "sha256:ff70335d468e7eb6ec65b95b99d3a2836546063f63acc5171de367e834932a81", size = 34031, upload-time = "2024-12-04T17:35:28.174Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/b7/ce/149a00dd41f10bc29e5921b496af8b574d8413afcd5e30dfa0ed46c2cc5e/six-1.17.0-py2.py3-none-any.whl", hash = "sha256:4721f391ed90541fddacab5acf947aa0d3dc7d27b2e1e8eda2be8970586c3274", size = 11050, upload-time = "2024-12-04T17:35:26.475Z" },
+]
+
 [[package]]
 name = "sqlparse"
 version = "0.5.5"