fix(store): default store_path when config omits it (issue #53)

Fresh installs shipped a config template with no store_path key while
collector.py demanded one via get_store_path() — every first collect
crashed with KeyError 'store_path'. Resolve to the packaged default
(/var/lib/fenris/observations.db) when absent, document the key in the
template, and cover the fresh-install path with regression tests.
Bump to 0.3.2.

src/fenris/store.py

@@ -15,9 +15,19 @@ from typing import Optional
 SCHEMA_VERSION = 1
 
 
+# Packaged default placement (spec §8.3). The config may override it, but a
+# fresh install that sets only the device selector must collect cleanly.
+DEFAULT_STORE_PATH = Path("/var/lib/fenris/observations.db")
+
+
 def get_store_path(config: dict) -> Path:
-    """Get the store path from config."""
-    return Path(config["store_path"])
+    """Get the store path from config.
+
+    Falls back to the packaged default when the config does not pin one,
+    so a fresh install whose config holds only the device selector works
+    instead of crashing with KeyError 'store_path' (issue #53).
+    """
+    return Path(config.get("store_path", DEFAULT_STORE_PATH))
 
 
 def init_store(store_path: Path) -> sqlite3.Connection: