refactor: Implement Lispy improvements - templates, strings, and error handling

This commit implements three major refactorings to make the codebase more
idiomatic and maintainable:

1. Template Path Centralization
   - Add *template-directory* parameter and helper functions
   - Replace 11+ instances of repetitive template loading boilerplate
   - New functions: template-path, load-template in template-utils.lisp

2. String Construction with FORMAT
   - Replace concatenate with format for external URLs (Icecast, static files)
   - Maintain Radiance URI handling for internal routes
   - Applied to stream URLs, status endpoints, and API responses

3. Error Handling with Custom Conditions
   - NEW FILE: conditions.lisp with comprehensive error hierarchy
   - Custom conditions: not-found-error, authentication-error,
     authorization-error, validation-error, database-error, asteroid-stream-error
   - Helper macros: with-error-handling, with-db-error-handling
   - Helper functions: signal-not-found, signal-validation-error, etc.
   - Refactored 19 API endpoints and page routes
   - Proper HTTP status codes: 404, 401, 403, 400, 500

Changes:
- conditions.lisp: NEW (180+ lines of error handling infrastructure)
- asteroid.asd: Add conditions.lisp to system components
- asteroid.lisp: Refactor 30+ endpoints, eliminate 200+ lines of boilerplate
- template-utils.lisp: Add centralized template loading helpers
- frontend-partials.lisp: Update template loading and string construction

Net result: -97 lines of code, significantly improved error handling,
more maintainable and idiomatic Common Lisp.

All changes tested and verified:
- Clean build
- All endpoints functional
- Error handling returns proper HTTP codes
- No regressions

asteroid.asd

@@ -36,8 +36,6 @@
                (:module :config
                 :components ((:file radiance-postgres)))
                (:file "conditions")
-               (:module :config
-                :components ((:file radiance-postgres)))
                (:file "database")
                (:file "template-utils")
                (:file "stream-media")