feat(vector): Add configurable chunk size and overlap for document embedding

Enable users to tune document chunking parameters to match their embedding
model and content type by adding DOCUMENT_CHUNK_SIZE and DOCUMENT_CHUNK_OVERLAP
environment variables.

- **config.py**: Added `document_chunk_size` (default: 512) and
  `document_chunk_overlap` (default: 50) configuration fields with validation:
  - Ensures overlap < chunk_size
  - Warns if chunk_size < 100 words
  - Prevents negative overlap values

- **processor.py**: Updated DocumentChunker instantiation to use config
  settings instead of hardcoded values (line 174-177)

- **tests/unit/test_config.py**: Added TestChunkConfigValidation class with
  9 tests covering:
  - Default values
  - Valid configurations
  - Validation errors (overlap >= chunk_size, negative overlap)
  - Warning for small chunk sizes
  - Environment variable loading

- **docs/configuration.md**: Added comprehensive "Document Chunking
  Configuration" section with:
  - Chunk size selection guidance (256-384 vs 512 vs 768-1024 words)
  - Overlap recommendations (10-20% of chunk size)
  - Configuration examples for different use cases
  - Added env vars to reference table

- **docs/semantic-search-architecture.md**: Added "Document Chunking Strategy"
  section with:
  - Chunking process explanation
  - Example showing sliding window behavior
  - Search behavior with chunks
  - Tuning recommendations

- **env.sample**: Added complete "Semantic Search & Vector Sync Configuration"
  section with:
  - Vector sync settings
  - Qdrant configuration (3 modes)
  - Ollama embedding service
  - Document chunking configuration

- **docker-compose.yml**: Added commented examples for DOCUMENT_CHUNK_SIZE and
  DOCUMENT_CHUNK_OVERLAP with usage notes

\`\`\`bash
DOCUMENT_CHUNK_SIZE=512

DOCUMENT_CHUNK_OVERLAP=50
\`\`\`

1. \`overlap\` must be less than \`chunk_size\`
2. \`overlap\` cannot be negative
3. Warning issued if \`chunk_size\` < 100 words

**Precise matching** (small notes, specific queries):
\`\`\`bash
DOCUMENT_CHUNK_SIZE=256
DOCUMENT_CHUNK_OVERLAP=25
\`\`\`

**Balanced** (default, general purpose):
\`\`\`bash
DOCUMENT_CHUNK_SIZE=512
DOCUMENT_CHUNK_OVERLAP=50
\`\`\`

**Contextual** (long documents, broader topics):
\`\`\`bash
DOCUMENT_CHUNK_SIZE=1024
DOCUMENT_CHUNK_OVERLAP=100
\`\`\`

✅ **User control** - Tune chunking to match embedding model capabilities
✅ **Experimentation** - Test different chunk sizes for optimal results
✅ **Model alignment** - Match chunk size to embedding context window
✅ **Backward compatible** - Defaults maintain existing behavior
✅ **Well validated** - Comprehensive tests prevent misconfiguration

All 22 config validation tests pass (9 new tests for chunking):
- Default values work correctly
- Validation prevents invalid configurations
- Environment variables load properly
- Warning system works as expected

With configurable chunk sizes, users can now experiment with different Ollama
embedding models and tune chunk parameters for optimal semantic search quality.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/semantic-search-architecture.md

@@ -177,6 +177,53 @@ Currently only `NotesScanner` is implemented. Future: `CalendarScanner`, `DeckSc
 - `user_id`: Multi-tenancy filtering (each user's vectors isolated)
 - `doc_type`: App identifier ("note", "event", "card", etc.)
 - `etag`: Change detection for incremental updates
+- `chunk_index`: Position of this chunk within the document (0-indexed)
+- `total_chunks`: Total number of chunks for this document
+- `excerpt`: First 200 characters of chunk (for display)
+
+### Document Chunking Strategy
+
+Documents are chunked before embedding to handle content larger than the embedding model's context window and to improve search precision.
+
+**Configuration:**
+```dotenv
+DOCUMENT_CHUNK_SIZE=512       # Words per chunk (default)
+DOCUMENT_CHUNK_OVERLAP=50     # Overlapping words between chunks (default)
+```
+
+**Chunking Process:**
+1. **Text combination**: Document title + content (e.g., `"Note Title\n\nNote content..."`)
+2. **Word-based splitting**: Simple whitespace tokenization
+3. **Sliding window**: Create overlapping chunks
+4. **Individual embedding**: Each chunk gets its own vector
+5. **Separate storage**: Each chunk stored as distinct point in Qdrant
+
+**Example:**
+```
+Document (1000 words):
+→ Chunk 0: words 0-511
+→ Chunk 1: words 462-973 (overlaps by 50 words)
+→ Chunk 2: words 924-999 (last chunk, partial)
+
+Each chunk stored as separate vector with metadata:
+- chunk_index: 0, 1, 2
+- total_chunks: 3
+- excerpt: First 200 chars of each chunk
+```
+
+**Search Behavior:**
+- **Vector search** operates on chunks (not whole documents)
+- **Deduplication** collapses multiple matching chunks from same document
+- **Best match** returns highest-scoring chunk's excerpt
+- **Access verification** still performed at document level
+
+**Tuning Recommendations:**
+- **Small chunks (256-384 words)**: More precise, less context, more storage
+- **Large chunks (768-1024 words)**: More context, less precise, less storage
+- **Overlap (10-20% of chunk size)**: Preserves context across boundaries
+- **Match to embedding model**: Consider model's context window when sizing
+
+**Important**: Changing chunk size requires re-embedding all documents. Use the collection naming strategy to manage different chunking configurations.
 
 ### Collection Naming and Model Switching