Closes M15 from issue #81 — SQL injection via attacker-controlled
identifiers in connectors/keboola/extractor.py and
connectors/bigquery/extractor.py.
Lifted _validate_identifier from src/orchestrator.py into a new
src/identifier_validation.py shared module (single source of truth for
both layers). Two validator policies:
- validate_identifier (strict, ^[a-zA-Z_][a-zA-Z0-9_]{0,63}$) for
table_name — matches the orchestrator's rebuild-time check, so dashed
names fail fast at extraction rather than being silently dropped.
- validate_quoted_identifier (relaxed, accepts dashes/dots) for
bucket/dataset/source_table — Keboola in.c-foo and BigQuery
my-dataset are legitimate, just need to be safe inside `"..."`.
Both extractors skip-and-continue on unsafe rows (logged + counted in
failure stats); _extract_via_extension re-validates as defense-in-depth.
71/71 extractor + orchestrator tests pass.
Refs #81 Group D.
Write to extract.duckdb.tmp, then atomically swap into place with WAL cleanup.
Prevents lock conflicts with orchestrator holding read lock on existing database.
DuckDB has used WAL by default since v0.8, so this pragma is not
valid DuckDB syntax. Removed obsolete try-except block that attempted
to enable WAL on system database initialization.
BigQuery extension handles auth via GOOGLE_APPLICATION_CREDENTIALS env var,
so _remote_attach uses empty token_env. Orchestrator now supports both
token-based (Keboola) and env-based (BigQuery) authentication modes.
