feat: add Database.merge() and sqlite-utils merge command

[abhiyadav2345]

Closes #491.

Adds the ability to merge tables from one or more source SQLite databases into a destination database.

Python API

# Merge all tables from two source databases
db.merge(["one.db", "two.db"])

# Add missing columns automatically
db.merge(["one.db", "two.db"], alter=True)

# Replace rows with conflicting primary keys
db.merge(["one.db"], replace=True)

# Skip rows with conflicting primary keys
db.merge(["one.db"], ignore=True)

# Only merge specific tables
db.merge(["one.db", "two.db"], tables=["cats", "dogs"])

# Source can be a Database object or a file path
db.merge([other_db, "another.db"])

CLI

sqlite-utils merge combined.db one.db two.db
sqlite-utils merge combined.db one.db two.db --alter
sqlite-utils merge combined.db one.db two.db --replace
sqlite-utils merge combined.db one.db two.db --ignore
sqlite-utils merge combined.db one.db two.db --table mytable

Implementation notes

• Tables not in the destination are created with the source schema and rows
• Tables that already exist have rows inserted (no-op if same rows with same PKs, unless --replace or --ignore)
• Primary keys are preserved from the source table by default; --pk overrides this
• Virtual tables (FTS indexes etc.) are skipped automatically
• FTS shadow tables (e.g. docs_fts_data) are also skipped via parent name detection
• merge() returns self for chaining

Tests

17 new tests covering the Python API and CLI — all passing, no regressions in existing 1042 tests.

Docs

Added a Merging databases section to docs/cli.rst with examples for all options.

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📚 Documentation preview 📚: https://sqlite-utils--724.org.readthedocs.build/en/724/