notifications: use "Estuary" in alert email subjects (#3000)

[0xfandom]

Description:

Alert emails prefixed their subject with Estuary Flow:, while the rest of the product says just "Estuary". This drops "Flow" from the subject of every alert type, and from the remaining body prose.

This mostly finishes a migration that was already underway — FreeTrial's fired subject was already Estuary Free Trial, MissingPaymentMethod's was already Estuary: …, and several bodies already read "Your Estuary account" / "Your Estuary {{spec_type}}".

Three commits, so the last two can be dropped independently if you'd rather scope this tightly:

1. Subjects — Estuary Flow: → Estuary: across all 13 subject templates. This is what Update product name in alert emails #3000 asks for.
2. Bodies — the free-trial and missing-payment-method bodies still read "We hope you are enjoying Estuary Flow". Prose rather than subject lines, but the same "cohesive messaging" point.
3. Tests — closes a coverage gap the rename exposed (see below).

Fixes #3000

Workflow steps:

No API or CLI surface changes; this only affects the subject and body text of outbound alert emails. For example, a failing capture previously sent:

Estuary Flow: Task failure detected for capture acmeCo/test/capture

and now sends:

Estuary: Task failure detected for capture acmeCo/test/capture

Affected alert types: AutoDiscoverFailed, BackgroundPublicationFailed, DataMovementStalled, FreeTrial, FreeTrialEnding, ShardFailed, TaskAutoDisabledFailing, TaskAutoDisabledIdle, TaskChronicallyFailing, TaskIdle.

Documentation links affected:

None. The subject strings live only in the notifications crate's handlebars templates; nothing under site/ documents them.

Notes for reviewers:

• A coverage gap turned up while doing this, and it's the part most worth a look. The subject templates for TaskIdle, TaskAutoDisabledFailing and TaskAutoDisabledIdle had no test asserting them — I changed one to BROKEN: and the suite stayed green, so the rename was touching three lines nothing guarded. The third commit adds a fired-state test per type, following the existing assert_email + snapshot pattern; the same deliberate break now fails on subject mismatch. All three are fired-only, so there's no resolved case. Happy to split that commit into its own PR if you'd prefer it separate from a rename.
• Snapshots: only the notifications__* files are updated. The parallel alert_notifications__* set appears orphaned — no test renders them, and they don't regenerate — and notifications: remove stale alert_notifications snapshot files #3079 is already removing them, so I left them alone rather than create a conflict. Each of the 5 updated snapshots is a one-line diff.
• free_trial's resolved subject is a conditional; its has_credit_card branch becomes Estuary: Paid Tier, which lines up with the existing else branch (Estuary Paid Tier: Enter Payment Info…).
• Verified with cargo test -p notifications — 28 passing (25 existing + 3 new). cargo fmt clean.

[aeluce]

Hey, thanks for the interest in contributing to Estuary!

I ended up wanting to change some other aspects of notification emails beyond the "Estuary vs. Estuary Flow" issue, so I opened my own PR to address #3000. Appreciate the thought, though!