feat: per-user/per-database override for client_idle_timeout

[mvidaurre]

Why

client_idle_timeout is currently global. That works for ordinary request/response clients, but it also disconnects intentionally quiet sessions such as LISTEN/NOTIFY subscribers. PostgreSQL settings cannot prevent this because PgDog applies the timeout to the frontend socket before PostgreSQL is involved.

This change makes the timeout configurable per user or logical database, so operators can exempt only long-lived listeners without weakening idle-client protection for everyone else.

Fixes #1462.

Configuration and precedence

Resolution follows the same precedence used by other PgDog overrides:

1. Matching user override
2. Logical database override
3. [general].client_idle_timeout

At every level, 0 means disabled for that client (Duration::MAX).

Exempt one user

# users.toml
[[users]]
name = "queue_listener"
database = "production"
password = "..."
client_idle_timeout = 0

Override one logical database

# pgdog.toml
[[databases]]
name = "production"
host = "primary.internal"
port = 5432
client_idle_timeout = 300_000

A logical database may have multiple [[databases]] entries for shards or replicas. The first configured non-None override is used across the logical database. Conflicting configured values produce a warning rather than depending silently on physical-entry ordering.

When user entries overlap through all_databases or databases, the last matching entry that configures client_idle_timeout wins. A later matching entry without this setting does not erase an earlier override; this preserves explicit policy when passthrough authentication appends a bare user entry.

Authenticated admin sessions always use the general timeout because the admin database is virtual and has no user or backend database configuration.

Implementation

• Adds client_idle_timeout: Option<u64> to User and Database, including database-URL query parameter support.
• Adds ConfigAndUsers::client_idle_timeout(user, database) to centralize precedence and 0 handling.
• Resolves timeout policy using the authenticated startup user and database in regular and mirror client paths, while retaining the authenticated admin flag across reloads.
• Keeps resolved timeouts on each client and tracks the configuration with a Weak<ConfigAndUsers>:
  • normal requests perform only an O(1) configuration identity check;
  • configuration reloads are picked up on the next buffer() invocation; an already pending idle read retains its current deadline until the client loop wakes;
  • old configuration snapshots are not kept alive by idle clients.
• Regenerates both checked-in JSON schemas.
• Uses a frontend-only loopback fixture for timeout tests, so they do not initialize backend pools or require PostgreSQL.
• Documents timeout precedence, reload behavior, and examples in docs/CLIENT_CONNECTION.md and the example configuration files.

Reviewer guide

Area	Files	What to review
Public configuration	pgdog-config/src/users.rs, pgdog-config/src/database.rs, pgdog-config/src/url.rs, .schema/*	New optional fields, URL parsing, and generated schemas
Resolution semantics	pgdog-config/src/core.rs	User/database/general precedence, duplicate physical database handling, conflict warning
Runtime integration	pgdog/src/frontend/client/timeouts.rs, pgdog/src/frontend/client/mod.rs	Timeout selection and reload-aware caching
Mirror path	pgdog/src/backend/pool/connection/mirror/mod.rs	Destination user/database resolution
Tests	pgdog/src/frontend/client/test/mod.rs, timeout/config unit tests	Exemption, precedence, admin reloads, snapshot lifetime, URL parsing, and frontend-only fixture

Compatibility

• Existing configurations without overrides retain the general timeout.
• client_idle_in_transaction_timeout and backend idle_timeout behavior are unchanged.
• This is explicit configuration; it does not attempt to infer active LISTEN registrations automatically.
• client_idle_timeout = 0 now consistently means disabled at general, database, and user levels.

Validation

• cargo fmt --all -- --check
• cargo nextest run --profile dev -p pgdog-config --lib — 124 passed
• Relevant pgdog timeout/routing tests via cargo nextest — 7 passed
• cargo clippy -p pgdog-config -p pgdog --all-targets — no warnings introduced by this PR
• JSON-schema regeneration is clean (cargo run -p pgdog-jsonschema)
• The branch was test-merged with current main; cargo check -p pgdog-config -p pgdog passed

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