feat: Implement Epic #678 - Payload Size Control with Server-Side Caching

Complete implementation of payload truncation and caching for all search types:

Stories Implemented:
- S1 (#679): Semantic search payload control with SQLite cache
- S2 (#680): FTS search payload control
- S3 (#681): Hybrid search payload control
- S4 (#682): Temporal search payload control
- S5 (#683): Multi-repo search payload control
- S6 (#684): Regex search payload control
- S7 (#685): SCIP queries payload control
- S8 (#686): File content default chunking

Key Features:
- SQLite-based cache with WAL mode for concurrent access
- UUID4 cache handles for content retrieval
- Preview truncation at 2000 chars (configurable)
- Paginated retrieval at 5000 chars/page
- TTL-based cleanup (15 min default)
- MCP tool: cidx-get-cached-content for retrieval
- REST endpoint: /api/cache/{handle}
- Web UI config for all payload cache settings
- Full parity between MCP and REST interfaces

Version bump: 8.4.46 → 8.5.0

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jsbattig

src/code_indexer/__init__.py

@@ -6,5 +6,5 @@
 HNSW graph indexing (O(log N) complexity).
 """
 
-__version__ = "8.4.46"
+__version__ = "8.5.0"
 __author__ = "Seba Battig"

src/code_indexer/server/app.py

@@ -3,6 +3,7 @@
 
 Story #526: Provides singleton HNSW index cache for server-wide performance optimization.
 Story #XXX: Provides singleton FTS (Tantivy) index cache for FTS query performance.
+Story #679: Provides PayloadCache for semantic search result truncation.
 """
 
 from code_indexer.server.middleware.correlation import get_correlation_id
@@ -18,6 +19,11 @@
     FTSIndexCacheEntry,
     FTSIndexCacheStats,
 )
+from .payload_cache import (
+    PayloadCache,
+    PayloadCacheConfig,
+    CacheNotFoundError,
+)
 
 # Server-wide singleton cache instances
 # Initialized on first import, shared across all server components
@@ -177,4 +183,8 @@ def reset_global_fts_cache() -> None:
     "FTSIndexCacheStats",
     "get_global_fts_cache",
     "reset_global_fts_cache",
+    # Payload cache exports (Story #679)
+    "PayloadCache",
+    "PayloadCacheConfig",
+    "CacheNotFoundError",
 ]