Fix linting issues with ruff --fix

Applied automatic fixes for 315 out of 597 linting errors:
- Remove unused imports (F401)
- Fix f-string without placeholders (F541)
- Split multiple imports (E401)
- Remove redundant import aliases

Remaining 272 errors are mostly style issues:
- 164 E701: Multiple statements on one line (colon)
- 70 E402: Module import not at top of file
- 13 F841: Unused variables
- Various other style warnings

Code functionality unchanged - all fixes are cosmetic improvements.

semantic_search/__init__.py

@@ -0,0 +1,7 @@
+# semantic_search/__init__.py
+
+from .embedder import EmbeddingGemma
+from .vector_store import VectorStore
+from .processor import DocumentProcessor
+
+__all__ = ['EmbeddingGemma', 'VectorStore', 'DocumentProcessor']