feat(notebooks): add block propagation by size analysis with regional breakdown (#50)

Adds a new notebook analyzing block propagation timing relative to block
size, with regional breakdown from two data sources.

### New notebook: Block propagation by size
(`09-block-propagation-size`)

Analyzes how block size affects propagation timing, comparing MEV vs
local blocks with corrected timing that isolates network propagation
from block building overhead.

**Key visualizations:**
- Size distribution by builder type (histogram)
- Compression ratio analysis
- MEV timing breakdown (building vs network time)
- Corrected first seen by size bucket (box plots)
- Regional propagation comparison (Sentries vs Contributoor)
- Regional CDF faceted by size bucket and builder type (4×2 grid)
- Propagation spread analysis
- Anomaly detection (z-score, residuals)

### Regional data sources

| Source | Description | Node count |
|--------|-------------|------------|
| **Sentries** | EthPandaOps libp2p gossipsub monitoring | ~50-100 |
| **Contributoor** | Community beacon API event collection | ~875 |

Both sources provide per-region (EU, NA, AS, OC) timing data, enabling
comparison of propagation patterns across geographic regions and
infrastructure types.

### New query modules

| File | Description |
|------|-------------|
| `queries/block_propagation_by_size.py` | Main propagation query +
Sentries regional query |
| `queries/block_propagation_contributoor.py` | Contributoor regional
query |

### Infrastructure changes

- **Multi-database ClickHouse support** in `scripts/fetch_data.py`
- New `get_client(database)` function routes queries to appropriate host
  - Per-query database selection via `database` field in pipeline.yaml
- **CI workflow updates** in `.github/workflows/sync.yml`
- Added `CONTRIBUTOOR_CLICKHOUSE_*` environment variables for
Contributoor queries

Author: raulk

.github/workflows/sync.yml

@@ -125,6 +125,10 @@ jobs:
           CLICKHOUSE_PORT: ${{ secrets.CLICKHOUSE_PORT }}
           CLICKHOUSE_USER: ${{ secrets.CLICKHOUSE_USER }}
           CLICKHOUSE_PASSWORD: ${{ secrets.CLICKHOUSE_PASSWORD }}
+          CONTRIBUTOOR_CLICKHOUSE_HOST: ${{ secrets.CONTRIBUTOOR_CLICKHOUSE_HOST }}
+          CONTRIBUTOOR_CLICKHOUSE_PORT: ${{ secrets.CONTRIBUTOOR_CLICKHOUSE_PORT }}
+          CONTRIBUTOOR_CLICKHOUSE_USER: ${{ secrets.CONTRIBUTOOR_CLICKHOUSE_USER }}
+          CONTRIBUTOOR_CLICKHOUSE_PASSWORD: ${{ secrets.CONTRIBUTOOR_CLICKHOUSE_PASSWORD }}
         run: just fetch all ${{ env.FORCE_RENDER }}
 
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