fix: address PR #317 code review feedback

**Critical Fixes:**
- Replace happy_path_error__with_fallback debug calls with proper logger methods in mcp-server.ts
- All HTTP API calls now use logger.debug/error for consistent logging

**Code Quality Improvements:**
- Extract 90-day recency window magic numbers to named constants
- Added RECENCY_WINDOW_DAYS and RECENCY_WINDOW_MS constants in SearchManager

**Documentation:**
- Document model cost implications of Haiku → Sonnet upgrade in CHANGELOG
- Provide clear migration path for users who want to revert to Haiku

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -24,6 +24,28 @@ Fixed critical bug where timeline tools were completely non-functional due to pa
 - New `get_batch_observations` MCP tool for efficiently fetching multiple observations in a single request
 - Enhanced SessionStore methods for fetching prompts and session summaries by ID
 
+### Changed
+
+**Default Model Upgrade: Haiku → Sonnet**
+
+The default model for observations and summaries has been upgraded from `claude-haiku-4-5` to `claude-sonnet-4-5` for significantly improved quality.
+
+**Impact on Costs:**
+- Sonnet is approximately 5-10x more expensive than Haiku
+- Better observation quality and more accurate context summaries
+- **Recommendation**: Review your usage patterns and consider adjusting `CLAUDE_MEM_MODEL` in `~/.claude-mem/settings.json` if cost is a concern
+
+**To revert to Haiku (lower cost):**
+```json
+{
+  "CLAUDE_MEM_MODEL": "claude-haiku-4-5"
+}
+```
+
+**Code Quality:**
+- Extracted magic numbers to constants (`RECENCY_WINDOW_DAYS`, `RECENCY_WINDOW_MS`)
+- Replaced debug logging calls with proper logger methods
+
 ---
 
 ## [7.2.1] - 2025-12-14