feat: Phase 5-6 — CREATE VIEW, migrations, deadlock detection, JSON logging, admin UI

Phase 5 — ERP Features:
- CREATE VIEW name AS SELECT ... — parser + executor with view expansion
- DROP VIEW — parser + executor
- CREATE MIGRATION name AS 'sql' — parser + executor, stored in LSM-Tree
- APPLY MIGRATION name — parser + executor, replays stored migration SQL
- Views table in ExecutionContext, expanded on SELECT FROM view

Phase 6 — Production Readiness:
- Deadlock detection: timeout-based auto-abort for stale transactions (30s default)
- TxnManager.txnTimeoutMs configurable
- Structured JSON logging module (logging.nim) with levels: debug/info/warn/error
- Admin dashboard Web UI at GET /admin — SQL playground, schema browser, metrics
- Dark theme HTML/CSS with tabs

Lexer: added tkView, tkMigration, tkApply tokens
Parser: parseCreateView, parseDropView, parseCreateMigration, parseApplyMigration
AST: nkCreateView, nkDropView, nkCreateMigration, nkApplyMigration nodes

All 216 tests pass

src/barabadb/core/logging.nim

@@ -0,0 +1,49 @@
+## BaraDB Structured JSON Logger
+import std/json
+import std/times
+import std/os
+
+type
+  LogLevel* = enum
+    llDebug = 0
+    llInfo = 1
+    llWarn = 2
+    llError = 3
+
+  Logger* = ref object
+    level*: LogLevel
+    output*: File
+
+var defaultLogger* = Logger(level: llInfo, output: stdout)
+
+proc newLogger*(level: LogLevel = llInfo, filepath: string = ""): Logger =
+  var f = stdout
+  if filepath.len > 0:
+    f = open(filepath, fmAppend)
+  Logger(level: level, output: f)
+
+proc log*(logger: Logger, level: LogLevel, msg: string, extra: JsonNode = newJNull()) =
+  if level < logger.level: return
+  let entry = %*{
+    "ts": $now(),
+    "level": $level,
+    "msg": msg,
+    "extra": extra
+  }
+  logger.output.writeLine($entry)
+  logger.output.flushFile()
+
+proc log*(msg: string, level: LogLevel = llInfo) =
+  defaultLogger.log(level, msg)
+
+proc debug*(msg: string) = defaultLogger.log(llDebug, msg)
+proc info*(msg: string) = defaultLogger.log(llInfo, msg)
+proc warn*(msg: string) = defaultLogger.log(llWarn, msg)
+proc errorMsg*(msg: string) = defaultLogger.log(llError, msg)
+
+proc setLevel*(logger: Logger, level: LogLevel) =
+  logger.level = level
+
+proc close*(logger: Logger) =
+  if logger.output != stdout:
+    logger.output.close()