fix: resolve CalDAV PROPFIND returning empty property values (#153)

Root cause: XML namespace prefixes (e.g. "D", "C") were not being
resolved to their actual namespace URIs (e.g. "DAV:",
"urn:ietf:params:xml:ns:caldav") during PROPFIND parsing. This caused
all property match arms to fall through to the catch-all, producing
empty XML elements.

Changes:
- Add namespace-aware XML parsing (collect_ns_decls + resolve_name) to
  WebDavAdapter, used by all DAV protocol parsers (WebDAV, CalDAV,
  CardDAV)
- Add /.well-known/caldav -> /caldav/ redirect (RFC 6764)
- Add root /caldav/ PROPFIND response with current-user-principal and
  calendar-home-set discovery properties
- Add /caldav/principals/{username}/ PROPFIND handler (was 500 error)
- Add /caldav/{username}/ user calendar home handler (calendar-home-set
  target)
- Respect Depth header at root: depth 0 returns only root entry, depth
  1+ includes calendar children
- Fix pre-existing TRANSCODE_POOL_THREADS test compilation error
- Add 8 new tests covering namespace resolution, discovery properties,
  and principal responses

https://claude.ai/code/session_01T49VBJSimgo28APxbucHzq

src/main.rs

@@ -114,6 +114,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     use oxicloud::interfaces::api::handlers::carddav_handler;
     use oxicloud::interfaces::api::handlers::webdav_handler;
     let caldav_router = caldav_handler::caldav_routes();
+    let well_known_router = caldav_handler::well_known_routes();
     let carddav_router = carddav_handler::carddav_routes();
     let webdav_router = webdav_handler::webdav_routes();
 
@@ -228,6 +229,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             .nest("/api", public_api_routes)
             // All other API routes are protected by auth middleware
             .nest("/api", protected_api)
+            // RFC 6764 well-known discovery (public, no auth — just redirects)
+            .merge(well_known_router.clone())
             // CalDAV/CardDAV/WebDAV protocols merged at top-level for client compatibility
             .merge(caldav_protected)
             .merge(carddav_protected)
@@ -251,6 +254,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         app = Router::new()
             .nest("/api", public_api_routes)
             .nest("/api", api_routes)
+            // RFC 6764 well-known discovery (just redirects)
+            .merge(well_known_router)
             // CalDAV/CardDAV/WebDAV protocols merged at top-level
             .merge(caldav_router)
             .merge(carddav_router)