Startup errors

Any configuration problem stops startup with exit code 1 and an error on stderr: a header line, then one bullet per problem. Loading runs in stages — read and parse the env files in order, then read, parse, interpolate, and schema-check the config files in order (stopping at the first broken file), check every setting value from every source, check the rules — and stops at the first stage that fails, reporting every problem in that stage.

Header	Cause
`DBMCP_CONFIG_FILE: empty entry in file list`	A file list has an empty entry (`a.yaml,,b.yaml`), checked before any file is read. The origin may also be `DBMCP_ENV_FILE` or `DBMCP_CONFIG_FILE in <env file>:<line>`; an empty flag value reports `--config-file: empty path` / `--env-file: empty path`
`failed to read config file <path> (from --config-file): <reason>`	A named file is missing, a directory, or unreadable (the origin is `--config-file`, `--env-file`, `DBMCP_CONFIG_FILE`, `DBMCP_ENV_FILE`, or `DBMCP_CONFIG_FILE in <env file>:<line>`; the env file variant starts with `failed to read env file`)
`failed to load env file <path>:`	Env file syntax errors, one `line N: …` bullet each
`failed to parse config file <path>:`	YAML syntax errors, with `file:line:column`
`failed to interpolate config file <path>:`	Malformed `${` or a failed `${VAR:?…}`
`invalid settings:`	Unknown keys and non-mapping sections in a config file, and values from any source with the wrong type (including strings that do not cast to an integer or boolean), unknown enum values, out-of-range integers, or empty list items. Values that lose to a higher-priority source are checked too
`configuration validation failed:`	Rule violations between valid values

Under invalid settings, each bullet starts with where the value is, then the dotted YAML key: /abs/dbmcp.yaml:2:14: for a config file, DBMCP_DB_PORT in /abs/.env:3: for an env file, or DBMCP_DB_PORT: for the process environment. Bullets list the config files first, then the env files, each in list order, then the process environment. A problem repeated by a file listed twice is reported once. Under configuration validation failed, each bullet names the setting by its dotted YAML key and ends with the source of the value: (from /abs/dbmcp.yaml:2:14), (from DBMCP_DB_PORT in /abs/.env:3), or (from DBMCP_DB_PORT).

Sample output:

$ DBMCP_DB_PORT=abc DBMCP_DB_CONNECTION_TIMEOUT=0 dbmcp --config-file /srv/dbmcp/dbmcp.yaml --env-file /srv/dbmcp/.env
invalid settings:
  - /srv/dbmcp/dbmcp.yaml:3:3: database.pasword: unknown field; expected one of backend, host, port, user, password, name, charset, ssl, ssl_ca, ssl_cert, ssl_key, ssl_verify_cert, read_only, max_pool_size, connection_timeout, query_timeout, max_query_timeout, page_size, max_cell_chars, max_response_chars
  - /srv/dbmcp/dbmcp.yaml:4:14: database.page_size: must be at least 1
  - DBMCP_DB_MAX_POOL_SIZE in /srv/dbmcp/.env:2: database.max_pool_size: must be at least 1
  - DBMCP_DB_CONNECTION_TIMEOUT: database.connection_timeout: must be at least 1
  - DBMCP_DB_PORT: database.port: expected non-negative integer, found string

$ DBMCP_DB_BACKEND=sqlite dbmcp --config-file /srv/dbmcp/dbmcp.yaml
configuration validation failed:
  - database.name (file path) is required for sqlite; set DBMCP_DB_NAME or database.name in the config file (from DBMCP_DB_BACKEND)

$ dbmcp --config-file /srv/dbmcp/dbmcp.yaml
failed to interpolate config file /srv/dbmcp/dbmcp.yaml:
  - /srv/dbmcp/dbmcp.yaml:2:13: database.password: required variable DB_PASSWORD is missing a value: set DB_PASSWORD

A removed per-setting flag is rejected by the argument parser with exit code 2 (error: unexpected argument '--db-backend' found). See Migrating from CLI flags for the variable that replaces each flag.