fix(M11): propagate PipelineCriticalError, redact configured secrets, signal blog failures

Correct 4 findings from the independent M11 review:

#1 (Critical) — PipelineCriticalError was downgraded to PipelineWarning:
  - Add except PipelineCriticalError: raise before each except Exception
    in all 5 non-blocking steps (fetch_blog, compare, caldav_sync, synthesis, send)
  - Critical errors now propagate to the outer handler and stop the pipeline

#2 (Critical) — redact_exception() did not use configured secrets:
  - Extend redact_exception() with extra_secrets parameter (upward compatible)
  - Harden redact_secrets(): sort extra_secrets by length descending
  - Add Settings.redaction_secrets() collecting all 6 SecretStr fields
  - Add PipelineRunner._redact(exc) using self._redaction_secrets
  - All except blocks in run() now use self._redact(exc)
  - CalDAV FAILED-status path uses full redaction_secrets collection

#3 (Medium) — BlogRSSClient silently swallowed failures:
  - Add error field to BlogRSSFetchResult
  - rss.py sets error on failure paths (except Exception, bozo/invalid feed)
  - fetch_blog_step raises RuntimeError when result.error is set
  - PipelineRunner now produces PipelineWarning for blog failures

#4 (Medium) — Test coverage at 80%, now 91%:
  - 11 new integration tests covering blog failure/success, compare failure,
    CalDAV failure (exception + FAILED status), send False/exception,
    PipelineCriticalError propagation, secret redaction with sentinel,
    empty agenda/homework, iCal cache cleanup
  - Secret redaction test uses mock (no network) and proves configured-secret
    propagation via non-URL sentinel in RuntimeError

Validation: 619 tests pass, ruff/mypy/bandit/pre-commit green, coverage 91%.

Co-authored-by: opencode/coder <coder@agents.invalid>
Co-authored-by: opencode/test-engineer <test-engineer@agents.invalid>

pronote_sync/sources/blog/rss.py

@@ -131,12 +131,14 @@ class BlogRSSClient:
             if getattr(feed, "bozo", None):
                 bozo_exception = getattr(feed, "bozo_exception", None)
                 if bozo_exception is not None:
+                    error_msg = f"Flux RSS invalide : {redact_exception(bozo_exception)}"
                     logger.warning(
                         "Flux RSS du blog invalide (%s), ignoré : %s",
                         redact_exception(bozo_exception),
                         redact_url(self.rss_url),
                     )
                 else:
+                    error_msg = "Flux RSS invalide"
                     logger.warning(
                         "Flux RSS du blog invalide, ignoré : %s",
                         redact_url(self.rss_url),
@@ -146,6 +148,7 @@ class BlogRSSClient:
                     etag=etag,
                     last_modified=last_modified,
                     not_modified=False,
+                    error=error_msg,
                 )
 
             articles: list[BlogArticle] = []
@@ -234,16 +237,18 @@ class BlogRSSClient:
                 not_modified=False,
             )
         except Exception as exc:
+            error_msg = redact_exception(exc)
             logger.error(
                 "Échec de la récupération du flux RSS du blog %s : %s",
                 redact_url(self.rss_url),
-                redact_exception(exc),
+                error_msg,
             )
             return BlogRSSFetchResult(
                 articles=(),
                 etag=etag,
                 last_modified=last_modified,
                 not_modified=False,
+                error=error_msg,
             )
 
     @staticmethod