perf: fix vector viz search performance and visual encoding

This commit addresses critical performance issues with vector visualization
search (reducing time from 40s to ~2s) and improves result visualization
through better visual encoding.

## Performance Fixes

### 1. Fix blocking sleep in retry decorator (base.py:51)
- Changed `time.sleep(5)` to `await anyio.sleep(5)` in @retry_on_429
- Prevents entire event loop from freezing during rate limit retries
- Impact: Reduced search time from 22s to 16s initially

### 2. Add concurrency limiting for verification (verification.py:77-93)
- Added `anyio.Semaphore(20)` to limit concurrent HTTP requests
- Prevents connection pool exhaustion (RequestError) from 90+ simultaneous requests
- Fixes false filtering (was filtering 77/90 results incorrectly)
- Note: Semaphore still in code but verification removed from viz endpoint

### 3. Remove unnecessary verification from viz endpoint (viz_routes.py:483-486)
- Visualization only needs Qdrant metadata (title, excerpt), not full content
- Verification only required for sampling (LLM needs full note content)
- Impact: Reduced search time from 43.7s to ~2s (final fix)

### 4. Restore streaming scanner pattern (scanner.py)
- Process notes one-at-a-time using async generator
- Avoids loading all notes into memory

## Visualization Improvements

### 5. Result-relative score normalization (viz_routes.py:489-504)
- Normalize scores within result set: best=1.0, worst=0.0
- Removes arbitrary RRF normalization (theoretical max didn't make sense)
- Makes visual encoding meaningful regardless of algorithm scores

### 6. Power scaling for marker sizes (userinfo_routes.py:743)
- Changed from linear `8 + (score * 12)` to power `6 + (score² * 14)`
- Creates dramatic visual contrast: 0.0→6px, 0.5→9.5px, 1.0→20px
- Combined with opacity (0.2-1.0) for clear visual hierarchy

### 7. Multi-channel visual encoding (userinfo_routes.py:740-745)
- Size: Exponentially scaled with score²
- Opacity: Linear 0.2-1.0 (keeps all points visible)
- Color: Viridis gradient (blue→yellow)
- Effect: Top results are large/bright/opaque, context results small/dim/transparent

## Result
- Search time: 40s → ~2s (20x faster)
- Visual contrast: Subtle → dramatic (clear result hierarchy)
- No arbitrary cutoffs: All results visible, best naturally highlighted

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

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

nextcloud_mcp_server/auth/userinfo_routes.py

@@ -725,6 +725,11 @@ async def user_info_html(request: Request) -> HTMLResponse:
                     }},
 
                     renderPlot(coordinates, results) {{
+                        // Calculate score range for auto-scaling
+                        const scores = results.map(r => r.score);
+                        const minScore = Math.min(...scores);
+                        const maxScore = Math.max(...scores);
+
                         const trace = {{
                             x: coordinates.map(c => c[0]),
                             y: coordinates.map(c => c[1]),
@@ -732,11 +737,18 @@ async def user_info_html(request: Request) -> HTMLResponse:
                             type: 'scatter',
                             text: results.map(r => `${{r.title}}<br>Score: ${{r.score.toFixed(3)}}`),
                             marker: {{
-                                size: 8,
-                                color: results.map(r => r.score),
+                                // Multi-channel encoding: size + opacity + color for visual hierarchy
+                                // Power scaling (score^2) amplifies visual differences dramatically
+                                // score=0.0 → 6px, score=0.5 → 9.5px, score=1.0 → 20px
+                                size: results.map(r => 6 + (Math.pow(r.score, 2) * 14)),
+                                // Linear opacity scaling (0.2-1.0 range keeps all points visible)
+                                opacity: results.map(r => 0.2 + (r.score * 0.8)),
+                                // Color gradient shows score
+                                color: scores,
                                 colorscale: 'Viridis',
                                 showscale: true,
-                                colorbar: {{ title: 'Score' }},
+                                colorbar: {{ title: 'Relative Score' }},
+                                // Scores are normalized 0-1 within result set
                                 cmin: 0,
                                 cmax: 1
                             }}