Fix high-priority bugs and add comprehensive test coverage

This commit addresses 8 high-priority issues identified in code analysis.

Fixes #3
Fixes #4
Fixes #5
Fixes #7
Fixes #10
Fixes #19
Fixes #20
Fixes #21

## Critical Bug Fixes

1. **Race condition in lock file creation (#3)**
   - Changed to atomic file creation using os.O_CREAT | os.O_EXCL
   - Prevents two processes from both acquiring the lock
   - Location: curateipsum/backup.py:110-115

2. **Invalid lock file error handling (#4)**
   - Added try/except for corrupted/empty lock files
   - Gracefully removes corrupted locks and retries
   - Location: curateipsum/backup.py:121-133

3. **SIGKILL vs SIGTERM issue (#5)**
   - Now sends SIGTERM first for graceful shutdown
   - Waits 5 seconds before escalating to SIGKILL
   - Allows previous process to clean up resources
   - Location: curateipsum/backup.py:146-156

4. **Wrong stat object for permissions (#7)**
   - Fixed bug where dst_stat was used instead of src_stat
   - Permissions are now correctly updated during rsync
   - Location: curateipsum/fs.py:371

5. **os.chown() fails for non-root users (#10)**
   - Wrapped all os.chown() calls in try/except blocks
   - Logs debug message instead of crashing
   - Allows backups to succeed for non-root users
   - Locations: curateipsum/fs.py:217-221, 228-231, 383-387, 469-472

## Comprehensive Test Coverage

6. **Lock file tests (#19)**
   - Added TestBackupLock class with 7 test cases
   - Tests: creation, concurrent prevention, stale locks, corruption
   - Location: tests/test_backups.py:228-330

7. **Filesystem operation tests (#20)**
   - Added tests/test_fs_extended.py with 6 test classes
   - Tests: copy_file, copy_direntry, rsync, hardlink_dir, scantree, rm_direntry
   - 20+ test cases covering normal and edge cases
   - Location: tests/test_fs_extended.py

8. **Integration tests (#21)**
   - Added tests/test_integration.py with 2 test classes
   - Tests full backup workflow end-to-end
   - Tests: incremental backups, hardlinks, delta dirs, cleanup, recovery
   - 14 test cases covering complete backup lifecycle
   - Location: tests/test_integration.py

## Test Results
All 68 tests pass successfully:
- 11 original backup cleanup tests
- 7 new lock file tests
- 16 original fs tests
- 20 new fs extended tests
- 14 new integration tests

## Impact
These fixes address critical bugs that could cause:
- Data corruption from concurrent backups
- Incomplete cleanup from forced process termination
- Permission sync failures
- Tool unusability for non-root users

The comprehensive test coverage ensures these bugs are caught early
and provides confidence for future refactoring.

curateipsum/fs.py

@@ -214,15 +214,21 @@ def copy_direntry(entry: Union[os.DirEntry, PseudoDirEntry], dst_path):
     if entry.is_symlink():
         # change symlink attributes only if supported by OS
         if os.chown in os.supports_follow_symlinks:
-            os.chown(dst_path, src_stat.st_uid, src_stat.st_gid,
-                     follow_symlinks=False)
+            try:
+                os.chown(dst_path, src_stat.st_uid, src_stat.st_gid,
+                         follow_symlinks=False)
+            except PermissionError:
+                _lg.debug("Cannot change ownership (not root): %s", dst_path)
         if os.chmod in os.supports_follow_symlinks:
             os.chmod(dst_path, src_stat.st_mode, follow_symlinks=False)
         if os.utime in os.supports_follow_symlinks:
             os.utime(dst_path, (src_stat.st_atime, src_stat.st_mtime),
                      follow_symlinks=False)
     else:
-        os.chown(dst_path, src_stat.st_uid, src_stat.st_gid)
+        try:
+            os.chown(dst_path, src_stat.st_uid, src_stat.st_gid)
+        except PermissionError:
+            _lg.debug("Cannot change ownership (not root): %s", dst_path)
         os.chmod(dst_path, src_stat.st_mode)
         os.utime(dst_path, (src_stat.st_atime, src_stat.st_mtime))
 
@@ -368,14 +374,17 @@ def rsync(src_dir,
         # update permissions and ownership
         if src_stat.st_mode != dst_stat.st_mode:
             _lg.debug("Rsync, updating permissions: %s", rel_path)
-            os.chmod(dst_entry.path, dst_stat.st_mode)
+            os.chmod(dst_entry.path, src_stat.st_mode)
             yield rel_path, Actions.UPDATE_PERM, ""
 
         if (src_stat.st_uid != dst_stat.st_uid
                 or src_stat.st_gid != dst_stat.st_gid):
             _lg.debug("Rsync, updating owners: %s", rel_path)
-            os.chown(dst_entry.path, src_stat.st_uid, src_stat.st_gid)
-            yield rel_path, Actions.UPDATE_OWNER, ""
+            try:
+                os.chown(dst_entry.path, src_stat.st_uid, src_stat.st_gid)
+                yield rel_path, Actions.UPDATE_OWNER, ""
+            except PermissionError:
+                _lg.debug("Cannot change ownership (not root): %s", rel_path)
 
     # process remained source entries (new files/dirs/symlinks)
     for rel_path, src_entry in src_files_map.items():
@@ -457,7 +466,10 @@ def _recursive_hardlink(src: str, dst: str) -> bool:
 
                 # save directory's metainfo
                 ent_stat = ent.stat(follow_symlinks=False)
-                os.chown(ent_dst_path, ent_stat.st_uid, ent_stat.st_gid)
+                try:
+                    os.chown(ent_dst_path, ent_stat.st_uid, ent_stat.st_gid)
+                except PermissionError:
+                    _lg.debug("Cannot change ownership (not root): %s", ent_dst_path)
                 os.chmod(ent_dst_path, ent_stat.st_mode)
                 os.utime(ent_dst_path, (ent_stat.st_atime, ent_stat.st_mtime))