Add DB table to store GTFS feed availability checks

[davidgamez]

Description

Availability should be derived from historical request attempts. It should not be stored directly on the feed or returned in the standard feed response.

Proposed solution

CREATE TABLE gtfsfeedavailabilitycheck (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),

    feed_id VARCHAR(255) NOT NULL,
    checked_at TIMESTAMPTZ NOT NULL DEFAULT now(),

    request_url TEXT NOT NULL,
    status_code INTEGER,
    latency_ms DOUBLE PRECISION,

    error_message TEXT,
    error_type VARCHAR(255),

    success BOOLEAN NOT NULL,

    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),

    CONSTRAINT gtfsfeedavailabilitycheck_feed_id_fkey
        FOREIGN KEY (feed_id)
        REFERENCES gtfsfeed(id)
        ON DELETE CASCADE
);

CREATE INDEX idx_gtfsfeedavailabilitycheck_feed_checked_at
    ON gtfsfeedavailabilitycheck (feed_id, checked_at DESC);

CREATE INDEX idx_gtfsfeedavailabilitycheck_checked_at
    ON gtfsfeedavailabilitycheck (checked_at DESC);

CREATE INDEX idx_gtfsfeedavailabilitycheck_feed_success_checked_at
    ON gtfsfeedavailabilitycheck (feed_id, success, checked_at DESC);

I would keep success even if it can be inferred from status_code, because it makes queries and chart aggregation easier.