Add Debian BTS source for bugs filed and owned

New `bts` subcommand backed by python-debianbts (bugs.debian.org SOAP):

- Identify by email address: get_bugs(submitter=...) for bugs filed,
  get_bugs(owner=...) for bugs owned
- Filed filtered on creation date, owned on log_modified; status
  lookups capped at 500 bugs per collect
- Per-query failures degrade to warnings like other sources

pyproject.toml

@@ -3,7 +3,12 @@ name = "weekly-activity"
 version = "0.2.0"
 description = "Weekly activity report (merge proposals, pull requests, bugs, issues) for Launchpad and GitHub."
 requires-python = ">=3.11"
-dependencies = ["launchpadlib>=2.1.0", "keyring>=25", "httpx>=0.27"]
+dependencies = [
+    "launchpadlib>=2.1.0",
+    "keyring>=25",
+    "httpx>=0.27",
+    "python-debianbts>=4.1.1",
+]
 
 [project.scripts]
 weekly-activity = "weekly_activity.cli:main"