fix: Add trailing slash to SITE_URL and enhance debug logging (v0.9.1)

Fix 1: SITE_URL trailing slash normalization
- IndieLogin.com requires client_id URLs to have trailing slash for root domains
- Added automatic normalization in load_config() after env loading
- Added secondary normalization after config overrides (for test compatibility)
- Fixes "client_id is not registered" authentication errors
- Updated redirect_uri construction to avoid double slashes

Fix 2: Enhanced httpx debug logging
- Added detailed request logging before token exchange POST
- Added detailed response logging after token exchange POST
- Shows exact HTTP method, URL, headers, and body for troubleshooting
- All sensitive data (tokens, verifiers) automatically redacted
- Supplements existing _log_http_request/_log_http_response helpers

Version: 0.9.1 (PATCH - bug fixes)
- Updated __version__ in starpunk/__init__.py
- Added CHANGELOG entry for v0.9.1

Tests: 486/514 passing (28 pre-existing failures from v0.8.0)
- No new test failures introduced
- Trailing slash normalization verified in config
- Debug logging outputs verified

Related: IndieLogin.com authentication flow
Following: docs/standards/git-branching-strategy.md

Generated with Claude Code

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

starpunk/config.py

@@ -20,7 +20,10 @@ def load_config(app, config_override=None):
     load_dotenv()
 
     # Site configuration
-    app.config["SITE_URL"] = os.getenv("SITE_URL", "http://localhost:5000")
+    # IndieWeb/OAuth specs require trailing slash for root URLs used as client_id
+    # See: https://indielogin.com/ OAuth client requirements
+    site_url = os.getenv("SITE_URL", "http://localhost:5000")
+    app.config["SITE_URL"] = site_url if site_url.endswith('/') else site_url + '/'
     app.config["SITE_NAME"] = os.getenv("SITE_NAME", "StarPunk")
     app.config["SITE_AUTHOR"] = os.getenv("SITE_AUTHOR", "Unknown")
     app.config["SITE_DESCRIPTION"] = os.getenv(
@@ -73,6 +76,11 @@ def load_config(app, config_override=None):
     if config_override:
         app.config.update(config_override)
 
+    # Normalize SITE_URL trailing slash (in case override provided URL without slash)
+    if "SITE_URL" in app.config:
+        site_url = app.config["SITE_URL"]
+        app.config["SITE_URL"] = site_url if site_url.endswith('/') else site_url + '/'
+
     # Convert path strings to Path objects (in case overrides provided strings)
     if isinstance(app.config["DATA_PATH"], str):
         app.config["DATA_PATH"] = Path(app.config["DATA_PATH"])