feat(mood): add the prediction table, with one writer and no writer yet

`MoodPrediction` holds the expected value of a day beside the day's own
rating: one row per account per local day, with a band and the number of
days the fit used. Two tables rather than four columns on the entry,
because the point of the second value is that the person cannot correct
it into agreement with the first — the distance between them is the
finding, and a value the edit form can write is not a second reading.

The table stays out of the disaster-recovery backup and the reason is
written where the completeness guard reads it: every row is recomputable
from the entries, the contexts and the linked modules, and carrying one
would restore a forecast made by a model version that may no longer
exist against data the restore has just moved. It is in the wipe plan,
because a derived row is still a day-by-day statement about somebody.

The write-surface guard lands red on purpose and names what is missing:
`lib/jobs/mood-prognosis-refresh.ts` does not exist yet, so the writer
set is empty where it should hold exactly that one file. The job closes
it. The rest of the guard is green now — no route, page or component
writes the table, and the matcher proves itself against a call broken
across lines, an aliased client, a read and a different model.

prisma/schema.prisma

@@ -774,6 +774,7 @@ model User {
   googleHealthOAuthStates        GoogleHealthOAuthState[]
   moodEntries                    MoodEntry[]
   moodContexts                   MoodContext[]
+  moodPredictions                MoodPrediction[]
   mentalHealthAssessments        MentalHealthAssessment[]
   moodTagsCustom                 MoodTag[]                       @relation("UserMoodTags")
   moodTagCategoriesCustom        MoodTagCategory[]               @relation("UserMoodTagCategories")
@@ -4256,6 +4257,68 @@ model MoodContext {
   @@map("mood_contexts")
 }
 
+/// What the account's own past days say a day like this one usually looks
+/// like — one row per local day, written by a job and never by a person.
+///
+/// Four things about this table that a later reader would otherwise have to
+/// infer, and would infer wrongly:
+///
+/// 1. **Written by the `mood-prognosis-refresh` job only. Never by a route,
+///    and the user cannot overwrite it.** The self-assessment on `MoodEntry`
+///    and the value here are two separate readings of the same day and they
+///    stay separate: the deviation between them is the entire finding, and a
+///    surface that let somebody correct the prognosis into agreement would
+///    destroy it. Two tables make that structural rather than a convention,
+///    and `src/__tests__/mood-prediction-write-surface-guard.test.ts` holds
+///    the writer set to one file.
+/// 2. **`features` holds the evidence, not a hint.** It is the JSON list of
+///    the standardised contributions the fit actually used for that day, so
+///    the "what the model weighted" line is generated from what happened
+///    rather than recomputed at render time against a model that may since
+///    have been re-fitted.
+/// 3. **Deliberately not in the disaster-recovery backup.** It is a derived
+///    artefact, recomputable in full from `MoodEntry` + `MoodContext` and the
+///    linked modules, and the nightly job rebuilds it after a restore. The
+///    verdict is written down in `DERIVED_MODELS`
+///    (`src/lib/export/backup-plan.ts`), where the completeness guard reads
+///    it, so this is a decision rather than an omission. It IS in the wipe
+///    plan: a derived row is still a statement about somebody's days.
+/// 4. **`n` and the band are part of the value.** A consumer that renders
+///    `predicted` without `ciLow` / `ciHigh` / `n` is a defect: a forecast
+///    built from thirty days and one built from three hundred look identical
+///    on screen and mean very different things.
+model MoodPrediction {
+  userId String @map("user_id")
+  /// YYYY-MM-DD in the account's own zone — the same day key and the same
+  /// anchoring `MoodEntry.date` carries, so the two readings of a day join
+  /// without a second timezone derivation.
+  date   String
+
+  /// The expected pleasantness (A1) for that day, 0-10, on the account's own
+  /// past pattern. A counterfactual, never a measurement.
+  predicted Float
+  /// The band around it, from the rolling-origin validation residuals. Null
+  /// only when the validation produced no residuals at all, which the write
+  /// path treats as "no forecast" rather than as a bare point estimate.
+  ciLow     Float? @map("ci_low")
+  ciHigh    Float? @map("ci_high")
+  /// Complete days the fit was built from. Shown wherever `predicted` is.
+  n         Int
+
+  /// Bumped whenever the feature set or the fit changes, so a re-fit is
+  /// legible in the stored rows instead of silently changing their meaning.
+  modelVersion String   @map("model_version")
+  /// JSON array of `{ feature, contribution }` — the standardised coefficient
+  /// times that day's standardised value, per feature.
+  features     String
+  computedAt   DateTime @map("computed_at")
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@id([userId, date])
+  @@map("mood_predictions")
+}
+
 // ─── Mood Tag Taxonomy (v1.8.5 W6-tax) ────────────────────────
 //
 // Structured, icon-bearing tag catalog modelled on the standalone mood