feat(kb): chunk_index in /kb/search hits + section-window in /source/{id}/chunks

Two minimal additions for the upcoming v0.1.11 client work:

- /kb/search hits now include chunk_index, so the client can call
  /source/{id}/chunks?around=N&ctx=K to fetch the matched chunk plus K
  chunks before/after as context. Required by the new search-preview
  layout for text sources, which until now duplicated the matched chunk
  in both panes.
- /source/{id}/chunks accepts around (chunk_index) + ctx (window radius
  clamped to 0..10). When around is given, returns only the windowed
  range; otherwise unchanged (returns all chunks of the source).

Refs Task Master #1

.gitignore

@@ -7,3 +7,27 @@ venv/
 .env
 *.log
 .pytest_cache/
+
+# Logs
+logs
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+dev-debug.log
+# Dependency directories
+node_modules/
+# Environment variables
+# Editor directories and files
+.idea
+.vscode
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?
+# OS specific
+.DS_Store
+
+# Task files
+# tasks.json
+# tasks/